dental materials science

LECTURE NOTES

DENTAL TECHNOLOGY DIPLOMA

Dental Material Science I

i Dental Material Science

General Introduction

Welcome to Dental Materials Science. Please have the notes with you during lectures, when the material will be further explained. Although you may find taking some notes is useful to give an extra view on some points, these notes cover all the material you will need to pass the module. You will not need to take any notes during the lecture, which will go over the material in these notes again by a Powerpoint presentation, or on the board. You may also find it useful to purchase the recommended materials science textbook, or to read the reference textbooks in the library. These books have been listed in the resources section which follows shortly. These notes have also been designed to allow you to study and learn the material away from class. Being able to cover the material at your own speed and with your own pattern of learning is beneficial for many students. To help with this, there are questions typical of some of those asked in the examinations at the end of each section. These allow you to check your knowledge of each section as you proceed.

Module Overview

This module has been designed to build your knowledge of atoms, material structure, chemical bonding, and properties of materials. This knowledge leads to a better understanding of the chemical, physical and mechanical properties of materials. In later sections of the course this knowledge will be valuable in understanding the reasons for using a particular dental restorative material, and the techniques necessary to fabricate it.

ii Dental Material Science

Recommended Textbook Title: Dental Materials properties and Manipulation. Author: R.C. Craig, J.M.Powers, & J.C. Wataha. Publisher: Mosby ISBN: 0-323-02520-X Reference Textbook Title: Dental Materials: Properties and Selection. Author: OBrien, William, J. Publisher: Quintessence Publishing Company Pub Place: Chicago Pub Date: 1989 ISBN: 0867151994 Your college may have a copy that you can borrow or you can purchase it yourself.

iii Dental Material Science

iv Dental Material Science

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COMM SERV HEALTH TOURISM HOSP ESD PARAMEDICAL STUDIES PROGRAM AREA

LEARNING OUTCOME 1

CLASSIFICATION OF MATTER. Assessment criteria: You will have achieved this learning outcome when you can:

Distinguish the different states of matter.

Classify matter as elements, mixtures or compounds.

TOPIC 1 - Classification of Matter.

Introduction In explaining to you the properties and use of various dental materials, we need first to understand what material, or matter, is. Scientists describe matter as belonging to different types in order to help understand its properties, For example, we often divide solid materials into metals, plastics (polymers) or ceramics, a classification we will learn more about later. Topic 1 introduces you to some of the ways scientists describe and classify matter. You will learn about:

States of matter (ways in which matter can exist)

Classification of matter as elements, mixtures or compounds.

Matter Matter is defined as anything which has mass and occupies a volume. Mass is the amount of material present. For example, when you see bubbles in a liquid, the bubbles have a volume, and the mass of the air can be determined. At the same time, you can observe differences between a gaseous and a liquid state of matter. This simple observation shows that the same matter can to exist in different states. Changing from one to another state of matter is a reversible, physical change. We can change water (a liquid form of hydrogen oxide) to ice (a solid form of the same compound) by cooling it sufficiently. We can change the ice back into water by heating it States of Matter Matter can exist in one of 4 forms:

(i) Solid. (ii) Liquid. (iii) Gas. (iv) Plasma

Plasma is a rare state of existence for matter on this planet and neednt bother us much for dental work, but it is important to understand the other three.

Recommended Time - 2 hrs

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The differences in the behaviour of matter as solid, liquid and gas is caused by the behaviour of its atoms in that state. For example, water can exist as a solid (ice) below 00C, as a liquid above 0oC, and as a gas if heated above 1000C. What causes the properties of these different states of matter is the how mobile the atoms are each state. In the solid state, the atoms or molecules are fixed in position due to strong forces between molecules. Because the molecules or atoms remain in these fixed positions, the only movement possible is vibration. Every solid has a fixed volume and a fixed, definite shape. However, when the solid is heated, the atoms or molecules react to the extra energy by vibrating with increased frequency and amplitude. They are still held firmly in place, however, and cannot break free of the forces holding them in place until their energy becomes great enough. At this point, the matter becomes a liquid. We say that it has reached its melting point. The forces between molecules are much weaker in liquids so the particles have greater mobility. Liquids are able to flow, a property due to the constant motion of their particles relative to one another. This is why they have no definite shape. The particles have only limited movement, however. They cannot move apart much, so that liquids have a constant, or fixed volume at any one temperature. Most particles of a liquid are held within the liquid due to forces of attraction between molecules (surface tension), but particles can gain enough energy to escape and form a vapour. This is called evaporation. As a liquid is heated, more and more particles evaporate until the temperature reaches the boiling point and a complete change of state from liquid to gas occurs. In the gaseous state the particles are in constant motion and free to move in any direction. As a result, gases are not only fluid (like liquids, they have no fixed shape), they also have no fixed volume. As the particles in a gas regularly collide and rebound from each other at high speed, they move around until they fill the whole container. If they are not contained they will fly away due to their highly mobile state. As gases are heated, the heat energy is transferred to increased motion (velocity) of the particles, and can be observed as an increase in pressure. At room temperature, if a substance appears as a liquid then it has a melting point below room temperature. If a substance is a gas at room temperature then it has a boiling point below room temperature. Kinetic Theory of Matter This theory explains how matter can be changed from one state to another. We know that all matter is be made up of tiny particles (atoms or molecules.) At any temperature above absolute zero, these particles are in a state of constant motion. The amount of motion of a substance in any state of matter is due to its particles responding to the available energy. Heat is a form of energy. If a substance is

liquid at room temperature, its particles are mobile, or in the liquid state. Its melting point is below room temperature. For water, with a melting point of 00C, there is enough energy at 200C to keep water particles mobile and able to flow. As the temperature of water is decreased the mobility of the molecules decrease until they cannot move fast enough to overcome the forces of attraction between them. The substance then changes state to become a solid. The molecules are no longer free to move, but they are still able to vibrate in positions fixed relative to each other. If we decrease the temperature further, particle vibration decreases until -2730C is reached. This is referred to absolute zero, since at this point all vibration stops. The temperature at which we observe changes of state are exact for pure substances. If we change the purity of the substance, the melting point and boiling point will also change. Homogenous and Heterogeneous Matter We use these terms describe the composition and properties of matter. For example, if we have pure water in a container, then a sample taken from anywhere in the liquid will have the same composition, and properties. Because of this, only one value of a property is needed to fully describe pure matter. It is said to be homogenous, its properties are identical at any point in it. If two elements or substances are mixed together, such as oil and water, they will quickly separate. We are able to easily separate the two distinct parts by physical means. This is called heterogeneous matter. The properties will not be the same at any point in the body. They will depend on which of the mixture components has been sampled. Elements, Mixtures and Compounds A pure substance is composed of only one type of atom or molecule, and it will have homogeneous properties. An atom is the smallest part of an element that still has the properties of that element A pure substance composed only of one sort of atom is called an element. A pure substance made from one sort of molecule is called a compound. A compound is a pure substance made by combining two or more elements in a fixed proportion by weight. A molecule is the smallest part of a compound that still has the properties of that compound When a second element is introduced to a first, the chemical and physical properties will change. For example, pure water melts at 00C and boils at 1000C.

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If we dissolve salt in the water, its composition changes. It is no longer pure. If we re-measure its melting and boiling points, they will have changed also. The difference in composition and properties between pure water and salt water explains the difference between a pure substance and a mixture. The properties of a mixture are a mixture of the properties of the individual substances. As the amounts of each substance vary, so will the properties. Mixtures do not have fixed melting points or boiling points; they change with composition. There is no chemical reaction involved in their formation so they are still easily separated. In the case of salt water, the water can be evaporated to form pure water leaving pure salt water behind. If two or more pure elements are made to react together chemically, then the result will be the formation of a compound. The compound formed will be a pure substance and will be different in composition and properties to the original elements. An example of forming a compound is the combustion of pure hydrogen in the presence of oxygen to form pure water. The compound formed is in a different state of matter at room temperature, does not resemble its gaseous reactants, and cannot easily be reversed to hydrogen and oxygen. A chemical reaction is needed to do that. Another example is mixing pure iron and sulphur together. Only when the mixture is heated will a chemical reaction occur to form a new compound, iron sulphide. This has different composition and properties to its reactants.

ELEMENTS, MIXTURES & COMPOUNDS Practical Assignment I

AIM To examine some of the properties of elements, mixtures and compounds. METHOD (1)

(i) Weigh out 5g of iron (Fe) and 3g of sulphur (S).

(ii) Place these two elements in a mortar and grind them into a fine powder with a pestle.

(iii) Spread this powder on a piece of filter paper and examine it with a magnifying glass. Is it uniform throughout (homogeneous) or is it non-uniform (heterogeneous)?

(iv) Pass a magnet under the filter paper and observe what happens. Is there any separation? Does this indicate a homogeneous or heterogeneous material?

(v) Fill a clean test tube to a depth of approximately 10 mm with the ground material and shake it up. Is there any separation? Half fill the test tube with water and shake. Is there any separation?

METHOD (2)

(i) Place 10mm of the ground material into a Pyrex test tube. Heat it over a bunsen flame in a fume cupboard. Note any reactions.

(ii) Allow the mass in the test tube to cool. Remove it form the test tube and grind it up in the mortar and pestle. Place some of the ground powder on a filter paper and examine it with a magnifying glass. Compare the results to those obtained in 1(iii).

(iii) Test the powder with a magnet as in 1(iv) and compare the results.

(iv) Place some of the material obtained in part 2 in a test tube to a depth of 10 mm and repeat 1(v). Compare the results.

RESULTS It would be helpful to record all your observations of the results in a tabulated form. CONCLUSIONS Consider the results you have written down. Do they indicate that we have an element, a mixture , or a compound at each stage of the experiment? What do the results show you about In light of your discussion draw some conclusions about elements, mixtures and compounds.

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Listed below are questions which will help you to review Topic 1, Write your answer to each question on the lines below the question. You can check your answers with the ones given at the end of this topic. Q1. List three (3) different states of matter.

(i) _____________________

(ii) _____________________

(iii) _____________________ Q2. Describe the motion of particle of matter in each state you listed in Q1.

(i) _____________________________________________________

(ii) _____________________________________________________

(iii) _____________________________________________________ Q3. Briefly describe the Kinetic Theory of Matter.

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

Q4. Define the following terms:

(i) Homogeneous

(ii) Heterogeneous

(iii) Element

(iv) Mixture

(v) Compound

Check Your Progress Self Evaluation Questions

Q1. (i) solid

(i) liquid

(ii) gas Q2. (i) Vibration of particles only rigidly restrained.

(ii) Particles have constant motion and can flow. (iii) Particles are free to move in any direction.

Q3. All matter consist of particles which are in a constant state of motion. Changes in temperature increase or decrease motion and cause changes of state.

Q4. Define the following terms:

(i) Homogeneous chemical properties are uniform only one physical distinct property throughout the

material.

(ii) Heterogeneous show two or more different property which allows separation easily by physical means

(iii) Element A pure substance composed of only one type of atom.

(iv) Mixture A substance containing with two or more elements or compounds combined in no fixed proportions. It shows the properties of each of its components.

The components are easily separated by physical means (heterogeneous)

(v) Compound This is a pure substance made by chemically combining atoms of two or more elements in fixed proportions by weight. A compound has its own chemical and physical properties, different from those of its components. It is still homogeneous.

Suggested Responses for Topic 1

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LEARNING OUTCOME 2

DISTINGUISH BETWEEN METALS AND NON-METALS, USING KNOWLEDGE OF

ATOMIC STRUCTURE. Assessment Criteria: You will have achieved this learning outcome when you can:

Name and describe sub-atomic particles.

Describe the arrangement of sub-atomic particles within the atom.

Relate the atomic number of an atom to its structure.

Classify atoms as metals or non-metals based on their structure.

Name, write the symbol for and describe the atomic structure of the first twelve elements.

Name, and write the symbol(s) for the elements within the periodic table that are of interest to dental restoration study.

TOPIC 2 - Atomic Structure.

Introduction We now understand that materials can be classified according to their different behaviour, both physical and chemical. To understand why materials are different from each other, we first must understand some basic chemistry about how matter is made, and how this influences its properties. This topic starts the process of understanding the basic structure of materials by introducing you to the structure of atoms, under the following headings:

The type and nature of sub atomic particles. The arrangement of sub atomic particles in an atom. The relationship between atomic number and structure. The classification of atoms, including the division into metals or non

metals. Important properties of the first twelve elements. Identifying elements that are present in dental materials.

History of Atomic Theory (just read this, not examinable) Precisely what goes to make up matter or substances is a problem that has fascinated scientific philosophers for centuries. Early philosophies considered matter to be made from of four elements: earth, fire, air and water. Around 400 BC Greek philosophers proposed that matter consisted of tiny indivisible particles called atoms. In the early 1800s John Dalton proposed a revolutionary new approach:

All elements are made up of atoms Atoms cannot be created or destroyed (they are indivisible) Atoms of different elements may combine with atoms of another

element in definite ratios. Atoms of one element are different from atoms of another element.

By 1820, laboratory experiment had found the presence of smaller particles in atoms, which suggested the presence of sub-atomic particles. Atoms could be taken apart, contrary to Daltons ideas.


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In 1911, Ernest Rutherford confirmed the presence of sub-atomic particles, and made the following conclusions:

Each atoms has a nucleus which is positively charged. Most of the atomic mass is contained in the nucleus. The nucleus is surrounded by an almost empty space that makes up the rest

of the atom. Negatively charged electrons are present in this space around the nucleus.

The negative charge on the electrons balances the positive charge of the nucleus.

In 1913, Niels Bohr suggested that the nucleus contains two different types of sub atomic particles. This gave rise to the modern atomic theory. Modern Atomic Theory Bohr conclude his atomic theory as:

Atoms consist of subatomic particles

The nucleus contains protons (+ charge) and neutrons (no charge).

A cloud of electrons (- charge) orbits the nucleus.

The volume of the nucleus is extremely small compared to the volume of an atom.

The atom is electrically neutral since the number of electrons = number of protons.

Properties of Subatomic Particles

Subatomic Particle

Symbol Charge Mass (grams)

Mass a.m.u

Location

Electron e- -1 9.07x10-28 0.00055 outside nucleus

Proton p+ +1 1.672x10-24 1.0073 inside nucleus

Neutron n 0 1.672x10-24 1.0087 inside nucleus

All elements are made up of different combinations of these subatomic particles. The number of each type of sub-atomic particle in an atom can be determined from the information about that particular element contained in the Periodic Table. This is a table of all the known elements and their basic properties, arranged in order of atomic number.

The arrangement of particles in an atom. PROTONS (+ charge), NEUTRONS (no charge) and ELECTRONS (- charge). Atomic Number and Mass Number What makes the difference between elements? To determine this we have to look at the arrangement of subatomic particles that make up each atom. This arrangement is different for each element. The information can be determined from the Atomic Number and the Mass Number:

Atomic number = number of protons in the nucleus Mass number = number of protons plus neutrons in the nucleus

As each atom must be electrically neutral, the number of electrons must be equal to the number of protons, which is the atomic number. If you look at the table of elements, you will see, for instance;

Atomic Number Mass Number Carbon 6 12

From this. we can work out that that carbon has:

6 protons (atomic number) 6 neutrons (mass number minus atomic number) 6 electrons (number of electrons = number of protons)

n n

e

e

e

e

n n

n p

n p

p n

Nucleus

Electron orbits

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Element Atomic Number (Z)

Mass Number (A)

Hydrogen 1 1 Helium 2 4 Lithium 3 7 Beryllium 4 9 Boron 5 11 Carbon 6 12 Nitrogen 7 14 Oxygen 8 16

We know now that the central nucleus contains the protons and neutrons, but we dont know how the electron orbits are arranged. For example, carbon has 6 protons (atomic number) and to be electrically neutral, must have 6 electrons. Logically there must be some consistent arrangement of these electrons, because they are moving around the nucleus without crashing into each other, Electron Structure Bohr described electrons as moving in fixed circular orbits around the nucleus, rather like planets orbiting around a sun. This is why his description is often referred to as the Planetary Model of an atom. These orbits, shells, or orbitals are different distances away from the nucleus, so each electron in a different shell must have a different energy. The electrons in the furthest orbit form the nucleus have the most energy, while those closer to the nucleus have less energy. Bohr identified each electron shell with a number, n. The shell closest to the nucleus had n=1 which is the lowest energy level. The next shell, n=2, has a higher energy level and so on for n=3,4,5,6. It seems logical that the outer shells, being larger, could hold more electrons. In fact, it turns out that the maximum number of electrons which can fit in each shell is governed by the formulae:

Maximum Number of electrons in any shell n = 2n2 n can be 1,2,3,4,5,6.

For example in the first electron shell, n=1.and n2 = 1. The maximum number of electrons which can be fitted into that shell is 2, because 2x(1)2=2. For the second shell n=2. n2 = 4 The maximum number of electrons in this shell is 8, because 2x(2)2 = 8 For the third shell n = 3. n2 = 9. The maximum number of electrons in the third shell is 18 because 2x(3)2 = 18

Consider the case of carbon. As we discussed earlier, it has six electrons. Its electron configuration can thus be calculated as

2 electrons in the 1st shell (n=1) 4 electrons in the 2nd shell (n=2)

The second shell could hold a maximum of eight electrons, but carbon only has six. After putting two into the first shell there are only four left, so the second shell can only have the remaining four electrons in it. Under normal conditions electrons in their shells are referred to as in their ground state. If atoms are heated, electrons gain energy and they may jump to higher energy levels. When dropping back to the ground state, they may re-emit the same amount of energy. If we gave each shell a number to identify it, this could become confused with the number of electrons in the shell, so chemists have identified each shell with a letter instead The closest shell to the nucleus is called K. The next is L followed by M, N etc. The following table shows the electron configuration for the first twelve elements. Remember, the maximum number of electrons is expressed by 2n2.

Element (chemical symbol)

Number of electrons

Maximum number in shell

Electron Configuration K , L , M , N

Hydrogen (H) 1 2 1

Helium (He) 2 2 2

Lithium (Li) 3 2, 8 2, 1

Beryllium (Be) 4 2, 8 2, 2

Boron ( B) 5 2, 8 2, 3

Carbon (C) 6 2, 8 2, 4

Nitrogen (N) 7 2, 8 2, 5

Oxygen (O) 8 2, 8 2, 6

Fluorine (F) 9 2, 8 2, 7

Neon (Ne) 10 2, 8 2, 8

Sodium (Na) 11 2, 8, 18 2, 8, 1

Magnesium (Mg) 12 2, 8, 18 2, 8, 2

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Names and symbols of elements As you can see from the table above, each different element has been given a name by its discoverer, and a symbol of one or two letters made up from the elements name or from its Latin name. The elements are named after greek or roman gods, scientists, countries, or anything else which took their discoverers fancy at the time. The symbols are used as a type of short-hand to represent the elements in chemical formulae and equations. For the purposes of this course, you should know the names and symbols of the first twelve elements, and their symbols, and those of another fourteen common elements or those of interest in dental work. These are set out in the next table Element Symbol Element Symbol Element Symbol Hydrogen H Gold Au Chlorine Cl Helium He Silver Ag Sulfur S Lithium Li Palladium Pd Phosphorus P Beryllium Be Platinum Pt Mercury Hg Boron B Iron Fe Zinc Zn Carbon C Cobalt Co Calcium Ca Nitrogen N Nickel Ni Oxygen O Chromium Cr Fluorine F Tin Sn Neon Ne Copper Cu Sodium Na Aluminium Al Magnesium Mg Lead Pb Periodic Table You will notice that the first twelve elements in the table have been listed in order of their Mass Number, which is the way that they were listed as the number of elements being discovered grew. The normal way of considering all the elements at once is the Periodic Table. Scientists in the 1800s discovered that some elements had very similar chemical properties to each other, even though their atomic and mass numbers were different. To find out why this was so, they arranged groups of elements with similar properties in columns and rows and looked for patterns in their properties or the numbers. In this way they made early versions of a periodic classification. The modern Periodic Table lists the elements in a series of boxes arranged in columns and rows.. In the horizontal row, elements increase in atomic number from left to right. Each box contains important information about the element:

Atomic number, or Z Mass number, or A Chemical symbol Electron configuration

Originally, elements with similar chemical properties had their boxes arranged in a vertical column. This arrangement is still used. For example, Helium, Neon, Argon, Krypton and Xenon are all inert gases and are found in a column on the right hand side of the periodic table. Look at the periodic table shown below. Although it is more than seventy years old, little has changed except for the addition of a few extra heavy elements with atomic weights above 105. Note that elements which have very obvious metallic properties are found on the left of the table. The less typical metals are found in the middle, the transition metals. Just before the non-metals on the right, separated by the stepped vertical line, there are elements which have some properties of both metals and non-metals, the metalloids such as arsenic, or silicon. Why should these obvious groupings occur? What determines the difference between metals and non-metals, so that arranging the elements in order of atomic number and similar chemical properties will reveal it? It turns out that the chemical properties of any element are controlled mainly by the number of electrons in its outer shells. This is a most important fact.

Properties of Metal and Non-Metals We now know, from a study of the types of chemical reactions they undergo, that what distinguishes metallic from non-metallic elements is the number of electrons they have in the outermost shell of their atoms. We call these valence electrons. An element will behave as a metal if it easily loses, or donates, one or more electrons when forming chemical bonds. Metals thus are those elements with

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only a few electrons in their outer shell. Look at the second column from the left in the periodic table. Use the atomic numbers to work out how many electrons each one has, and youll find they all have two electrons in their outermost shell. Non-Metals are elements that have outer electron shells that are close to being full. They readily accept electrons during chemical bonding. A comparison of the common properties of metals and non metals is shown in the following table: Metal Non Metal

Have 1, 2, or 3 valence electrons.

Lose electrons easily. Form compounds with non

metals. High electrical conductivity. High thermal conductivity. Malleable and Ductile.

Have 4, or more valence electrons.

Tend to gain electrons. Form compounds with metals. Low electrical conductivity. Low thermal conductivity. Non ductile (brittle).

With a self-paced learning package like this one, we provide regular opportunities for you to check your knowledge as you go. Youll find a set of questions for you to answer, and so review your knowledge, at the end of each topic. Listed below are questions that will help you to review Topic 2. Write your answer to each question on the lines below the question, and when you have finished you can check your answers with the ones given at the end of this topic. Q1 Give the name and charge of each of the particles that make up an atom

(i)

(i) (ii)

(ii) .

Q2 Define what is meant by the following terms. (i) Atomic Number ..

(ii) Mass Number ..

...

Q3 The maximum number of electrons in any shell with number n is calculated

from which formula?.

Q4. List three (3) properties of metals and non metals.

Metals Non Metals

(i) (i)

(ii) (ii)

(iii) (iii)


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Q1 Give the name and charge of each of the particles that go to make up an atom.

(i) proton (+) (ii) electron (-) (iii) neutron (0)

Q2. Complete the following sentences:.

(iii) The Atomic Number of an atom is The number of protons in the nucleus.

(iv) The Mass Number of an atom is The number of protons plus the number of neutrons in the nucleus.

Q3. The maximum number of electrons in any shell with number n is calculated

from the formula :- Maximum number =2n2 where n = shell number Q4. List three (3) properties of metals and non-metals.

Metals Non-Metals

(i) 1,2 or 3 valence electrons (i) 4 or more valence electrons

(ii) lose electrons easily (ii) tend to gain electrons

(iii) malleable (iii) brittle


So, how did you go with the topics under Learning outcome 2? Did you answer all the questions correctly? Do you feel confident in being able to meet the assessment criteria listed under the learning outcome? If you did, congratulations and please proceed to Learning outcome 2. If you answered a question incorrectly or you had difficulty with any of the activities, go back and have a look at the information again. If any part of this module is not clear, it is very important to contact your teacher and discuss this with him or her before you start Learning outcome 3.

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LEARNING OUTCOME 3

RELATE CHEMICAL REACTIONS AND BONDING TO THE STRUCTURE OF

MATTER. Assessment criteria: You will have achieved this learning outcome when you can:

Describe primary and secondary bonding in matter.

Classify the bonding in different materials.

Write balanced chemical equations to describe chemical reactions.

TOPIC 3 - Chemical Bonding and Reactions

Introduction The way that elements are combined to make compounds often has much to do with their properties as possible dental materials. This topic introduces you to important information about: Nature and types of primary and secondary bonds in matter. Specific types of bonding in materials.

Writing and balancing chemical equation to represent reactions. If you think about what might happen as two atoms are brought closer together, it is obvious that what they do will be controlled by their outer electron shells, which are the first parts to come together. We now know that atoms will be in a lower state of energy if their outer electron shells are full of electrons. If they are close to another atom, they can achieve this condition by obtaining extra electrons from it and filling their outer shell, or by giving electrons to it, so as to empty their outer shell and expose the full shell next to it, or by sharing electrons with the other atom so that both have a full outer shell. When atoms do this, they very often bond together to form a new substance, a compound Valency The combining power of an atom is known as its valency, or valence, and is an important property of each different element. It is determined by the number of electrons the atom will acquire, give away, or share during chemical bonding. electrons, as these high energy electrons are the ones involved in chemical bonding to from compounds. Metallic atoms have 1, 2, or 3 valence electrons which they lose easily during bonding to form a positive ion. An Ion is an atom that has been given a positive or negative charge by losing or gaining electrons. Non metallic atoms tend to gain electrons during bonding, so the number of vacant site in the outermost shell is their valency. Examples are shown in the following table. These elements form negative ions. Also in the table are a


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number of negative ions and one positive ion which are made from a number of atoms, not one. These negative or positive groups of atoms are called functional groups, because they can function as, or take the place of, a single atom or ion.

Positive valency ions Negative valency ions

Lithium Li1+ Carbon C4-

Beryllium Be2+ Chloride CI1-

Boron B3+ Carbonate CO32-

Sodium Na1+ Fluoride FI3-

Magnesium Mg2+ Hydroxide OH1-

Aluminium AI3+ Oxide O2-

Potassium K1+ Nitrate NO31-

Calcium Ca2+ Sulfate SO42-

Hydrogen H1+ Phosphate PO43-

Iron Fe2+ or Fe3+

Copper Cu1+ or Cu2+

Silver Ag1+

Gold Au1+

Ammonium NH41+

Chemical Bonding in Compounds The simplest form of bonding which occurs to form a compound is shown by non-metals such as oxygen, hydrogen or nitrogen. Two gas atoms like these can bond together to produce a stable diatomic molecule. The chemical formulae shows this:

Oxygen - O2 Chlorine - CI2 Nitrogen - N2 Hydrogen - H2

For bonding to occur between metals and non-metals, an ion may have to be formed. This is a charged atom, an atom with some electrons missing, or some extra electrons added. For example, aluminium has 3 valence electrons, but bonding is not possible until an ion has been formed by removing these.

Al (element) Al3+ + 3 electrons Na (element) Na1+ + 1 electron

Diatomic gases

An ion of aluminium has been formed form the element by losing 3 electrons. Similarly Sodium loses 1 electron to form an ion. As an example of a non-metal, the element chlorine can gain one electron to form a chloride ion for bonding.

Cl (element) + e Cl1- Ions of opposite charges attract each other, and may join together to form an ionic compound. An example of an ionic compound is sodium chloride

Na1+ + Cl1- NaCl The ions involved are Na1+ and Cl1- As they both have the same valency they are going to react in a 1:1 ratio, that is one ion of sodium (Na1+) will react with one chloride ion (Cl1-) to form the new compound NaCl. Another example is the reaction between zinc ions (Zn2+) and chloride ions (Cl1-). In this case the valency of zinc allows it to react with two chloride ions Zinc has two electrons to lose, and each chlorine atom can only pick up one of them, so the ratio of Zinc ions to Chlorine ions in the new compound is 1:2.

(Zn2+) + Cl1- ZnCl2 Notice how we use the abbreviated symbols given to the elements to describe the chemical reaction and also to construct a formula for the new compounds formed. How do these ions, in their fixed ratios, come together to form what we see as large amounts of matter, material, or substances? To answer this question, we must discuss chemical bonding, what holds atoms together Primary Chemical Bonds 1. Ionic Bonds

The stability and strength of these bonds can be explained by looking at the formation of sodium chloride we discussed above. Sodium has 1 electron in its outer shell which it will freely give up so that its next innermost shell is full, a more stable arrangement. Chlorine needs 1 electron to fill its most outermost shell, similarly becoming more stable. When these two elements react, electrons are taken from the sodium to the chloride atoms. The positive sodium ions and negative chloride ions formed have a strong electrostatic attraction to each other, and are held in place in a fixed structure to from a solid compound (at room temperature.)

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This newly formed compound, sodium chloride, normal table salt, is a solid, normally small clear crystals. Its properties are very different from those of the reactive shiny metal which is sodium and the green poisonous gas, chlorine. This difference proves the formation of a new substance. Ionic compounds such as sodium chloride have high melting points, are usually solid at room temperature and are soluble in water. They conduct electricity when they melt or when dissolved in water because they separate into ions (dissociate) which can carry an electric current. Steps in ionic bond formation: i) formation of sodium ion Na Na1+ + 1 electron ii) chlorine accept electron Cl + 1e CI1- iii) elecrostatic attraction forms new

compound Na+ + Cl- NaCI

2. Covalent Bonds

Atoms may achieve stable electron configurations by sharing electrons with adjacent atoms rather than donating or accepting them like elements which form ionic bonds. The gases we discussed earlier oxygen, hydrogen and carbon, all share electrons to fill their outer shells, which makes them more stable. Sharing outer shell electrons between atoms makes a new outer shell which encloses both atoms. A molecule of a new substance is created, with its atoms held together by a covalent bond. For example, consider a hydrogen atom with its single electron.

Two hydrogen atoms pair together to become a diatomic. molecule. To do this, they share their two electrons, which form a single orbital around both atoms. The orbital has a figure-eight shape.

We can write this as an equation, showing the reagent atoms and their product molecule.

H. + H. HH (hydrogen gas molecule, formula H2) Other non metallic atoms can share atoms to form covalently bonded molecules. For example a molecule of water (formula, H2O) is formed from two atoms of hydrogen and one of oxygen.

H. + H + O HOH (water) The bond which holds the atoms together in a molecule forms because the positive nucleus of each atom is strongly attracted to the cloud of shared electrons surrounding the atoms. A more extensive example of covalent bonding is found in methane, CH4. Here carbon, shares each of its four outer shell electrons in a bond with one of four separate hydrogen atoms. This results in eight electrons in a complex orbital around the carbon, and two around each hydrogen atom. All the atoms have an effectively full outer shell. The structural diagram for this is simpler, but does not show each electron. H |

HCH | H

H O H

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Atoms undergoing covalent bonding may also share more than 2 electrons. Sharing four electrons, for example, produces a double bond An example can be seen in the gas ethylene. There is a double bond between the two carbon atoms, which also share an electron in turn to each of two hydro atoms. Each carbon atom has thus shared its outer four electrons with another four from different atoms to give a total number of eight in its outer shell. The ethylene molecule forms a very reactive gas. H H \ /

C=C / \ H H The atomic or electronic diagram shows us the structure of each bond

H

H H

H

C

3. Metallic Bonds

This type of bonding is only formed between metallic atoms. There are not enough valence electrons in metals to share between atoms to make a true covalent bond. So, each atom contributes its valence electron to form a loose cloud of electrons. These electrons are not associated with any of the positive metallic ions formed, but are free to move between them. This produces a strong electrostatic attraction between the positively charged ions and the cloud of mobile electrons. The strength of the metallic bond produces very close packing of the ions in a regular lattice arrangement. The freely moving electron cloud produces good electrical conductivity, and the close packing gives metals their typically high densities.

C C

H H

H H

Atoms

Electron cloud

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4. Secondary Chemical Bonds

Secondary bonds are formed by the attraction of weak intermolecular forces between dipoles. A dipole is part of a covalently bonded molecule where charge is not evenly distributed. An example is water. The hydrogen atoms are not bonded to form a linear molecule, in fact the bond angle between the hydrogen is only 104.50 degrees instead of 180 degrees. This produces a weakly positive electrostatic charge on one side of the molecule and a weak negative charge on the opposite side. Attraction is now possible between oppositely charged poles of other polar molecules.

Chemical Reactions and Equations During the previous paragraphs, we have been coming close to describing the reaction between elements to from new compounds not just in words, but also by using the symbols which are the shorthand identification for the elements. Now we will expand this idea so that you can learn how scientists describe complicated chemical reactions in a brief and precise way. Chemical reactions involve substances which are present before the reaction. These substances are called reactants, or reagents The reaction produces new substances formed in a chemical change which are called products. There are many different types of chemical reactions, but for our purposes we only need to consider the general type described by the following form. Two or more elements, or one or more compounds (the Reagents) react or break down to make one or more different elements or compounds (the Products)

REAGENT + REAGENT PRODUCT + PRODUCT

Balancing Chemical Equations

H

O

104. 50 +

H

Chemical equations can tell us not only what elements and compounds are involved, but also how much of each one is needed, and how much is produced. For them to do this, they need to be Balanced. We say that an equation is balanced if it tells us tell us the exact truth about the reaction. It must show the correct reagents and products, and the correct amounts of each one. We test for a balanced equation by using the principal that we cannot make (or lose) atoms. Therefore exactly the same number of atoms of each element involved should be present in the reagent compounds as is in the product compounds. Where atoms combine in different ratios the correct number of atoms is important to complete the reaction. Balancing is achieved as a number of steps:

1. Write the equation for the reaction in words, making sure that the correct reagents and products are specified. In many cases, such as your examinations, the equation will be given to you already in symbols, in which case you cam leave out this step.

2. Express the products and reactants as the correct chemical symbols and formulae, and that the correct formulae are used for each one.

3. Count the number of atoms of each type of element in the reagents and in the products.

4. Balance the elements one at a time, using as many steps as needed. Example Hydrogen burns in oxygen to make water Step 1 Oxygen + Hydrogen Water Step 2 O2 + H2 H2O

Note The formula for oxygen and hydrogen are correct; we know that these gases are normally present as diatomic molecules. Also, the formula for water is correct, we know it is made up from two atoms of hydrogen and one atom of oxygen Remember the molecular formulae for these in previous pages?

Step 3 Note. There are two atoms of hydrogen in the reagents and in the

product. The equation is balanced for hydrogen. There are two atoms of oxygen in the reagents, but only one atom of oxygen in the products. The equation is not balanced for oxygen

Step 4A O2 + H2 2H2O

Note. Now we have doubled the number of water molecules produced, we have balanced the equation for oxygen (two atoms in reagents and products) but it is unbalanced for hydrogen.

Step 4B O2 + 2H2 2H2O

Note. Now both oxygen and hydrogen are balanced. There are two atoms of oxygen and four atoms of hydrogen on both sides of the equation.

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The equation now shows the same number of each type of atom among the reagents as among the products. We say that it is a balanced equation. Note that we cannot change the small numbers below the line in each formula, such as the 2 in H2O. These numbers are part of the formula; they show us the exact numbers of each atom in a molecule of a compound. To change them would be to change the compound into another one. For example, we could change H2O to H2O2, but this would be changing the compound from water to hydrogen peroxide, an entirely different compound! What we have to do is increase or reduce the number of molecules of each compound, or atoms of each free element until the equation balances. We show this variation by altering the large numbers placed before the formula for the compound or element. For example, H2O means one molecule of water, containing two hydrogen atoms and one oxygen atom. 2H2O means two molecules of water, therefore four atoms of hydrogen and two atoms of oxygen. For clarity, these numbers are bold, italic, and bigger in the equations above.

Example: Sodium metal reacts with water

Step 1: Sodium+ water Sodium Hydroxide + hydrogen gas

Step 2: Na + H2O NaOH + H2 Step 2 Note. There is one atom of sodium on each side of the equation,

and one atom of oxygen. But there are two toms of hydrogen in the reagents and three in the products.

Step 4A: Na +2H2O NaOH + H2 Step 4B Na + 2H2O 2NaOH + H2 Step 4C 2Na + 2H2O NaOH + H2 Now we will try a harder equation to balance: Balance the reaction for calcium hydroxide and nitric acid.

Step 1: Nitric acid + Calcium Hydroxide Calcium nitrate + water

Step 2: HNO3 + Ca(OH)2 Ca(NO3)2 + H2O

Step 3 Note The reagents have 3xH, 5xO, 1xN, 1xCa. The products have 2xH, 7xO, 1xN, 1xCa

Step 4A: 2HNO3 + Ca(OH)2 Ca(NO3)2 + H2O Note. this balances the N, but H and O are still unbalanced.

Step 4B 2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O Note4xH, 2xN, 1xCa, 8x0 4xH, 2xN, 1xCa, 8x0

Same number of atoms on both side. it balances!

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Try balancing the following equations. The answers are at the end of this unit.

1. C2H6 + O2 CO2 + H2O

2. Ca + HCI CaCI2 + H2

3. KOH + AI(NO3)3 KNO3 + AI(OH)3

4. H2SO4 + AI(OH)3 H2O + AI2(SO4)3 5. FeCI2 + Na3PO4 Fe3(PO4)2 + NaCI 6. CaCO3 + H3PO4 Ca3(PO4)2 + H2O + CO2 7 Mg + O2 MgO

Activity 1 Student Exercise

1. 2C2H6 + 7O2 4CO2 + 6H2O 2. Ca + 2HCI H2 + CaCI2 3. 3KOH + AI(NO3)3 3KNO3 + AI(OH)3 4. 3H2SO4 + 2AI(OH)3 6H2O + AI2(SO4)3 5. 3FeCI2 + 2Na3PO4 Fe3(PO4)2 + 6NaCI 6. 3CaCO3 + 2H3PO4 Ca3(PO4)2 + 3H2O + 3CO2 7. Mg + O2 MgO

Activity 1 - Answersheet

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PHYSICAL & CHEMICAL CHANGE = Assignment 2 AIM To observe and describe changes of properties of various materials and to classify them as:

a) physical b) chemical c) allotropic change

An allotropic change is a special form of physical change where the material changes its solid state structure, that is, the arrangement of atoms used to put it together. METHOD 1. (i) Dissolve a small amount of ammonium dichromate in water in a test

tube. Record what is happening. Retain the solution.

(ii) Ignite a small quantity of dry ammonium dichromate in an evaporating basin in a fume cupboard. Note any reactions, sparks evolved, colour changes or volume changes. Dissolve some of the reaction product in water and compare with 1 (i).

2. (i) Gently heat (in a fume cupboard) a small quantity of napthalene in a crucible until melting occurs. Allow to cool, and note all changes.

(ii) Reheat strongly (in a fume cupboard) projecting the flame down onto the material. Note all reactions.

3. (i) Place a lump of limestone (calcium carbonate) in a test tube and add a few drops of water. Note any reactions.

(iii) Add a few drops of hydrochloric acid to the test tube in 3(i). Note any reactions.

4. (i) Dissolve some salt (sodium chloride) in 5 ml water. Evaporate to dryness. Note the changes that have occurred.

5. (i) Heat (in a fume cupboard), a test tube 1/3 full of sulphur. Note any changes in state, viscosity and colour as the sulphur reacts to the heating. Note each change carefully. (Do not heat to strongly as sulphur will ignite).

(ii) Warm the sides of the test tube and pour the ,molten sulphur into a beaker of water. Compare the properties of the solid produced with the original sulphur. Note the changes that have occurred.

RESULTS Record all the results in tabulated form. Include chemical equations where applicable. CONCLUSION To come to some conclusions about what has happened, you need to look at the results you observed, and decide whether there was a physical reaction, a chemical reaction, or an allotropic reaction in each case.

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Listed below are questions which will help you to review Topic 3, Write your answer to each question on the lines below the question. You can check your answer with the one given at the end of this topic. Q1. Fill in the missing words (i) Metallic atoms have valence electrons.

(ii) Non metallic atoms have valence electrons. Q2. How do metallic ions form from a metal atom? (i) Metallic ions are formed by

.. (ii) Give an example: .

Q3. Write an equation to represent an ionic bond. Q4. What type of force holds ionic compounds such as sodium chloride together?

Q5. Covalent bonds are characterized by . Q6. Write an equation which gives an example of covalent bonding.

..

Q7. Describe metallic bonding.

..

.. Q8 Describe Secondary bonds... .. Q9. Balance this equation: C3H2COOH + O2 CO2 + H2O


Q1. (i) 1, 2 or 3.

(ii) 4 or more. Q2. (i) Losing their valence electrons to gain a positive valence.

(ii) Al Al3+ + 3e Q3. Sodium plus chlorine salt

Na+ + CI- NaCI

Q4. A very strong electrostatic attraction between the positive sodium ion and the negative chloride ion.

Q5. Sharing electrons to fill the outer shell.

Q6. H. + .O. + .H H:O:H (water)

Q7. The positively charged metallic ions are strongly attracted to a freely moving negative electron cloud.

Q8 Weak intermolecular forces of attraction between oppositely charged dipoles. Q9. C3H2 COOH + 5O2 4CO2 + 4H2O


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LEARNING OUTCOME 4

MATERIALS CLASSIFICATION, ALLOYS, POLYMERS OR CERAMICS.

Assessment criteria: You will have achieved this learning outcome when you can:

Describe the bonding in alloys, polymers or ceramics.

Categorise common dental materials as alloys, polymers or ceramics.

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TOPIC 4 - Structure of Materials

Introduction Dental applications involve some of the most fascinating and varied materials used nowadays, but, since no one type of materials possess all the desired properties for a particular dental application, we have to use a range of different materials, or combinations of them. . Later in the Diploma of Dental Technology the modules you study will include discussions of the properties and uses of Dental Metals, Dental Polymers, and Dental Ceramics. First, however, we need an introduction to these groups of materials, which we will find in this topic. This topic introduces you to important basic information about:

The bonding which constructs alloys, polymers and ceramics.

Classifying dental materials as alloys, polymers or ceramics.

Classification of Materials Materials used in dental applications can be divided into 4 families:

(i) metals and their alloys (ii) ceramics and glasses (iii) low and high molecular weight polymers and elastomers (iv) composites

Each of these material groups has specific properties which make them useful as dental materials. There is also great variation for these properties. For example, metals and ceramics show very limited flexibility (they are comparatively stiff, or rigid,) whereas polymers can be compounded to give the rubbery behaviour necessary in impression materials. Metals (metallic bonding) We can explain the chemical behaviour of materials by the number of their outer shell electrons. The mechanical properties of materials are caused by the way that

Recommended Time - 1 hr

atoms are arranged in order to make bulk material, We call this arrangement the structure of the material. The mechanical and physical properties of metals can be explained in terms of their metallic bonding. As discussed earlier in this unit, positive metallic ions are held rigidly in a close packed crystalline lattice structure, around which an electron cloud freely moves. This electron cloud can transmit energy with little loss, and so produces the characteristic metallic properties of electrical and thermal conductivity. If light is shone on a metal surface, it is reflected, producing a characteristic lustre. Most metals have high melting points, which is explained by the strength of the metallic bond. The ions need much heat energy to overcome electrostatic bonding forces and break free of each other, change state and become liquid. In terms of mechanical properties, most metals are as tough and ductile. They can be stressed below a certain limit and return to their original dimensions when the stress is released (elastic behaviour) or they can be stressed above their elastic limit and become permanently deformed (this plastic behaviour makes them formable, a useful property). Deformation without fracture is possible because layers of the crystalline arrangements can slip past one another under stress, another property of the metallic bond structure. Polymers (covalent bonding) Organic materials involve covalent bonding which involves sharing electrons They commonly form large molecules or macromolecules.by a process of repeated joining of a basic group of atoms, called polymerisation. These molecules may be many hundreds of thousands of atoms in size. In may polymers the molecules take the form of long chains of atoms, where the atoms are joined by covalent bonding, but the chains are only held to each other by weaker secondary bonds. Other polymers may have very large three-dimensional structures of atoms. When polymers melt the molecules separate from one another and move independently. Polymer melting points are much lower than those of metals or ceramics because only secondary bonds need to be broken. For the same reason. the strengths of polymers will also be much lower than that of metals or ceramics. . The rigidity of polymers is also lower than that of metals or ceramics. However, the low weight of most of the atoms in polymer molecules, and their relatively large spaces between chains makes polymers much less denseThe bonding of polymer atoms in chains or rings is strong, but the secondary bonds between chains are weak unless there is covalently bonded crosslinking. Increased temperature causes separation of the chains to allow each one to vibrate more. This phenomenon gives polymers much higher thermal expansion than metals or ceramics. Also, water can penetrate the weak bonding between chains, producing a susceptibility to swelling and degradation.

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Ceramics (ionic bonding) A simple definition of a ceramic is, A compound of metal ions and non-metal ions with ionic bonding, although there are some ceramics, such as glasses, with covalent bonding. A simpler definition of a ceramic is a material whose structure has been caused by firing it. Either way, their high strength bonds make ceramics very stable, with high melting points and rigidity (stiffness.) They are poor conductors, since the electrons donated by metal ions are strongly held by the non-metallic ion. On melting the crystal structure of the compound separates making the ions mobile, so ceramics can carry electric currents. when molten. Ceramics are also characterised by their high hardness and brittleness, as well as outstanding resistance to high temperatures. These properties are due to the electron behaviour of the constituent ions. Also due to this electron behaviour, ceramics are usually electrical and thermal insulators. Ceramics may be transparent (window glass) or coloured by absorption of ions as well as the suspension of pigments. This property is again due to the absence of free electrons in the material. Colouring is important to dental ceramics as their final shading must be matched to the patients natural tooth colour and stains. Hardening and strengthening of ceramics is possible by incorporating other ionic compounds like alumina (A1203). This increases their strength and rigidity. A big disadvantage of ceramics is their brittleness. Unlike metals, and a number of polymers, the strong ionic bond and crystalline structure does not allow any localised movement under stress. At higher stresses, the only response possible from the structure is to fracture. A brittle failure results, with a sudden release of energy. Many ceramic articles are made by firing powder until the particles fuse, because the ceramic melting points are inconveniently high for casting, unlike many metals. The spaces between the particles results in small entrapped voids which are hard to remove. Ceramics are poor heat conductors, so the outer surface cools faster, placing it under a tensile stress which produces small cracks. Although very strong in compression, the combination of surface cracks and internal defects like porosity makes ceramics much weaker in tension than many metals. As a result, ceramics are most suitable for use in applications where they are stressed in compression. Ceramics for dental applications are selected for their high strength and ease of processing. Their brittleness can be improved by forming a type of composite material when they are fused to the surface of a reinforcing metal structure which has a much greater toughness. Composites (covalent bonding matrix metallic or ionically bonded fillers). A composite is a material with two or more distinct phases. One phase is usually much harder than the other, and more brittle. The softer phase is generally

tougher. The combination of materials has properties that each of the separate ones cannot match. For example, human teeth can be repaired by a composite of a setting acrylic polymer with hard glass (ceramic) particles mixed in with it. This gives the composite the following properties:

(i) good strength and toughness (ii) good bonding to natural teeth (iii) wear resistance (iv) rapid setting (polymerised by light) (v) ease of use

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Listed below are questions which will help you to review Topic 4. Write your answer to each question on the lines below the question. You can check your answer with the script section at the end of this topic. Q1. List four (4) families of material (i) ______________________________

(ii) ______________________________ (iii) ______________________________ (iv) ______________________________

Q2. List three (3) properties of metals

(i) ______________________________ (ii) ______________________________ (iii) ______________________________

Q3. Define what is meant y a polymer ___________________________________________________

______________________________________________________________ Q4. List three (3) properties of polymers

(i) _____________________________ (ii) _____________________________ (iii) _____________________________

Q5. Crosslinking is _______________________________________________ Q6. Ceramics are _______________________ bonded. Q7. List 3 properties of ceramics

(i) _____________________________ (ii) _____________________________ (iii) _____________________________

Q8. The biggest disadvantage of ceramics is their ______________________ which is due to ______________________________________________ Q9. Composites have ______ phases, one of which is ____________ and the

other __________


Q1. i) metals and alloys

ii) ceramics and glasses iii) polymers and elastomers iv) composites

Q2. i) high melting point

ii) metallic bonding iii) high density

Q3. organic materials characterised by long chains which are covalently

bonded Q4. i) low density

ii) low softening points iii) covalent bonding

Q5. chemical bonding between polymer chains Q6. ionically Q7. i) poor electrical conductors

ii) brittle iii) high hardness

Q8. brittleness, surface cracks and internal defects Q9. two, hard, soft


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LEARNING OUTCOME 5

RATIONALISE THE USE OF SELECTED MATERIALS ACCORDING TO THEIR

PROPERTIES. Assessment criteria: You will have achieved this learning outcome when you can:

Classify the properties of materials as chemical, physical or mechanical.

Relate selected properties of materials to their use in dental technology applications.

Define and calculate specific properties using data for selected dental materials.

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TOPIC 5 - Properties of Materials

Introduction Now that we know about the basic differences between types of material, we need to start looking at their properties. Selection of materials for dental uses involves matching these properties against those needed for the particular application. In order to specify what properties we need, we must start by considering what properties there are, and which ones are relevant to the particular use. This topic introduces you to important basic information about:

Classification of chemical, physical and mechanical properties useful for dental applications.

Properties which are desired in various dental materials.

Calculating specific properties from data obtained by testing dental materials.

Property Classification The properties of materials can be classified into three categories:

(i) chemical (ii) physical (iii) mechanical

(i) Chemical Properties The chemical properties of a material describe the types of chemical reactions that it will undergo in various circumstances. Some physical properties are also linked to the chemical structure of the material.

The chemical properties of elements include valency and reactivity., stability, corrosion resistance, acidity or alkalinity, and composition. Of particular dental importance, are the reactions the material may have with human tissue.


(ii) Physical Properties The physical properties of a substance are describe how it reacts to the physical universe. Often, these properties are used to identify a substance. Examples are boiling point, melting point, electrical conductivity, or density.

(iii) Mechanical Properties These properties describe how substances react to applied forces. They are often measured by destructive testing, such as by tensile, compressive or impact tests. Typical properties include tensile or compressive strengths, stiffness, (rigidity) hardness, brittleness, fatigue resistance and impact toughness. These properties are usually related to how the atoms are arranged in a substance. Before looking at the properties needed for specific dental applications, we need to examine some of them more fully. (i) Bonding - The adhesion of one substance to another is an important chemical

property. For example, if they cannot bond to the natural tooth, then restorative materials would be useless.

Wetting - The wetting characteristics determine if molecules are compatible

in terms of their bonding energy. Good wetting produces high strength bonding. The surface tension of the material is a measure of this energy. If materials like solders, enamels or adhesives do not wet a surface, they will not join to it.

Stability - Material in the oral environment must not react or change in any

way which alters its properties. If the material absorbs fluid and swells, or is attacked by oral fluids then its usefulness is limited.

Toxicity Ideally dental materials are fully bio-compatible That is, they

can become integrated with human tissue without unfavourable results. Any material used certainly should not cause trauma or tissue damage to the patient. Care must be taken when some degradation may occur, or where certain materials may be toxic or reactive to patient or technician if used in certain ways.

Chemical Properties

(ii) Dimensional -The material should not shrink or contract and so cause discomfort Stability for the patient and so diminished usefulness of the appliance.

Physical Properties

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Rheology - This is the behaviour of materials moving under stress, such as

when mixing, or when being squeezed out of a tube. The flow and viscosity of some materials used to make dental appliances are also important.

Density - This is the mass per unit volume of a material. Polymers have low

densities and metals have high densities. This property needs consideration when selecting materials, as it controls the weight of an appliance. It may also control how much the material costs

Melting Point - The temperature at which a substance changes from solid to liquid

state. It is important in determining how easy a metal is to cast, because it partly determines the energy needed for melting it.

Boiling Point - This is the temperature at which a substance changes from liquid to

gaseous states. It will affect how a material is to be processed, or how it may react in dental use.

Optical Properties - There are a number of these, such as colour (accurately defined and

quantified), opacity and reflectance. These determine how light is transmitted, absorbed, or changed in wavelength on meeting the material. This controls its appearance, which is dentally important. These properties are vitally important to dental ceramics.

Coefficient Of Thermal Expansion - Almost all substances expand as their temperature rises, and

contract if cooled. As a result they change size with temperature, and the amount can be significant to the accuracy of metal castings, polymer restorations, and ceramics. In a mixture of two different materials (such as dental metal/ceramic restorations) two different rates of expansion can set up destructive stresses on cooling

Comparison of the varying amounts by which different materials expand is given by their coefficient of expansion. This is the amount a material will expand for every degree Celsius its temperature rises and for every unit of its original length. So, dental chromium alloys have a coefficient of linear expansion, (called ). of 15 x 10-6 mm per mm of original length, per oC rise in temperature. We use the formula: L1 = Lo + Lo x x T, where L1 = Final length, after expansion or contraction Lo = Original length, before expansion or contraction = coefficient of linear expansion of the materials T = Change in temperature

Substances also have expansion coefficients of area () and volume () To illustrate the importance of this property, consider a dental chromium/cobalt alloy partial denture casting measuring 75mm across. This would solidify at 1425oC, and after solidifying it will contract by 1.6mm (15 x 10-6 x 1400 x 75 mm) as it cools to room temperature. Unless some allowance is made for this (which is done, as you will learn in the Module Removable Alloy Partial Dentures) such a casting can become significantly undersized.

Specific heat This is a measure of the capacity of a material to absorb energy,

while changing temperature. It is measured as the energy required to raise the temperature of one unit of mass of the substance by one degree. So, for example, calories per gram per degree Celsius (Cal/g/oC) Metal with high specific heat takes longer heating (more energy) from a gas torch to reach the same temperature as a metal with lower specific heat.

Viscosity This is a measure of the resistance to flow of a fluid. The unit in

the metric system is the Poise. For example, a fluid like honey has a higher viscosity than water. Fluids with high viscosity flow less under pressure. An example where the viscosity is important in dental work is where the fluid must be poured to shape, such as an impression material.

Surface Tension This can be a difficult property to understand, but is of vital

importance in dental work whenever a fluid needs to wet and flow over a surface. Surface tension is a measure of the force with which one material is attracted to another at their surface, and is related to the chemical phenomenon of wetting.

Look at the diagrams below. An atom within the body of a liquid; it is evenly surrounded by other atoms of the same type. Their attraction for it is evenly distributed, which holds the atom in one place. On the surface of the liquid, however, the same atom is partly surrounded by atoms in the surface of whatever substance the liquid is touching. The force on the atom is now unbalanced; it may be attracted towards the new material surface, or back into the liquid, depending on how much it is attracted to atoms in the new surface. This imbalance means that the liquid will be attracted to, or repelled by a different material. Such a force mean that liquids will wet a new surface such as a solid, and spread across it, or they will not wet it, and withdraw from it.

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The results of surface tension are seen in the capacity of surfaces to pull liquid upwards against gravity. Examples are liquid soaking into finely porous material, or in sap rising in trees

Latent Heat This is the amount of energy, expressed in calories per gram, which a substance takes up or gives off when it changes state. So, there is a latent heat of fusion, needed to melt material already at its melting point, and a latent heat of vaporisation, needed to boil material already at its boiling point.

Material with a high specific heat, high latent heat of fusion, and

high melting point, needs much more energy to melt it, say for casting, than material with lower values for these properties

(iii)

To give satisfactory service in the oral environment, any material needs to have sufficient strength to withstand the stresses involved. A number of properties like the strength of a material can be measured from a Tensile Test. To carry out this test, we take a material specimen of suitable size. We measure the length to be tested, and the cross-sectional area at right angles to the applied force. We apply a gradually increasing force to the material, and measure the increase in length which occurs and the increasing force needed to continue extending the material. The test machine will gives us an applied force, measured in Newtons in the metric system, and an extension of the specimen, in millimetres. Before describing the results from such a test, we need to consider the units we are measuring. We need to understand exactly what a Force, and a Stress are.

Mechanical Properties

An atom in the centre is pulled evenly in all directions by other atoms

An atom at the surface is pulled inwards or outwards. This creates a surface tension

A force is defined as applying energy to a substance, so that it tends to move. Applying a force, f, to a body of mass m, produces an acceleration, a

F = m x a A Force can be applied to a body in a number of ways. If we apply a linear force, the body is in tension, or in compression

TENSION, A TENSILE FORCE

COMPRESSION, A COMPRESSIVE FORCE Other forces include bending, twisting and shear forces A BENDING FORCE TORSION, A TWISTING FORCE SHEAR, A PARTING FORCE

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A typical test graph for a tensile test is shown on the next page, but what is plotted on the diagram is not the force and extension the test machine measured, but the related properties of stress and strain. Stress is defined as the force applied to a substance, divided by the area of material across which the force is applied. Strain is the resulting change in size of the body of material (say, its extra length) divided by the original size We need stress and strain because they are properties of the material independent of the size of the specimen used. Consider two steel bolts, one with a cross-section area of ten square millimetres, and one with a cross-sectional area of twenty square millimetres. It will take twice as much force to break the bigger bolt in a tensile test, not because the steel in it is stronger, but because there is twice as much of it. But if we divide the force used to break each bolt by the area of the bolt, we will get the identical stress.. The unit of stress in the metric system is the Pascal, which is defined as a Newton of force per square metre of area. Unfortunately, the Pascal is such a small amount of stress that we commonly find ourselves measuring things in millions of Pascals, or MegaPascals Conveniently for our test, if we measure the force in Newtons, and the area of cross-section of the specimen in square millimetres, the resulting stress comes out in MegaPascals. Similarly, we want to measure as a material property, not the extension produced, but the strain, or percentage extension, which wont vary with the original length of the specimen. A B It takes twice as much force to break piece A than piece B, but if we divide the force by the cross-section area of the piece, the resulting stress experience by the material is the same

The next diagram is a graph of the stress against strain that happens when a piece of material is subjected to a tensile force Maximum, or Ultimate Stress Elastic limit C Breaking Stress or B D Yield Point S t r e s s A Elastic region Plastic region % Elongation Now, let us see what the important areas on this diagram can tell us about the mechanical properties of the material tested,

Elastic Limit This is sometimes called the Yield Point of the material, point B on

the diagram It is the stress at which the material stops behaving elastically and starts behaving plastically. That means that, if we were to remove the test force below the point B, the material would return to its original length (behave elastically) After the stress at point B, if we remove the applied force, the material will not return to its original length. It will have become plastically, or permanently deformed. So, the Yield Point or Elastic Limit, is the value of the stress at point B, is a measure of how easy it is to deform (say bend, or stretch) the material.

Elastic Modulus - Alternatively called stiffness or rigidity or the modulus of

rigidity, or Youngs Modulus, after its inventor, it describes how much material may elastically distort in use. In our diagram, the rigidity of a material is shown by the slope of the elastic part of the

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test line, A-B. The rigidity would be found from the stress at the point B divided by the strain at the point B.

In the diagrams below, a) shows the stress/strain relationship for a

material of high rigidity, such as a chromium alloy, whereas b) shows the curve for a material of low rigidity, such as a polymer.

The rigidity of a material is a measure of how stiff it is, how

much it resists deflection stress stress strain strain

a) High rigidity material b) Low rigidity material

Ultimate Tensile Strength - This is the maximum stress a material can withstand, the stress at

point C on the test diagram. The Ultimate Tensile Stress or U.T.S. is a measure of how much

stress a material will carry before it breaks. This is what is called strength

Plasticity - Often called ductility, this is a measure of the amount of

deformation a material can withstand up to its failure at point D. Brittle materials, such as glass have low plasticity, whereas ductile materials have high plasticity. A completely brittle material would have a stress/strain diagram like b) below, whereas a ductile material would show a curve like a), and give a large value of strain at the breaking point

Ductility is thus a measure of how much we can change the shape

of a material before it breaks.

stress stress strain at breaking strain strain a) Stress/ strain curve for b) Stress/strain curve for ductile material brittle material (no plastic deformation Resilience - This is the ability of a material to store elastic energy and release it

when the force is removed. This can be considered as the area under the elastic part of the curve, shaded in the diagram below. Resilience is the amount of energy a material can store before deforming, obviously an important property for assessing material for making springs

resilience stress strain Toughness- This the amount of energy a material can absorb without fracture.

On a stress/strain diagram it is represented by the total area under the curve. Toughness can be seen as the total amount of energy a material will store before breaking

stress toughness strain Fatigue - This is the resistance of a material to failure by repeated stressing

and unloading at a level of force lower than is required to break the

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material by one application e.g. mastication. Think of it as cumulative damage

Hardness - Hardness is related to wear resistance, an important property which

can control the useful life of a material in dental application. Hardness is defined as the resistance of a substance to indentation. It is measured by making a test indentation in the substance using an indenter of specific shape and size, and applying a set load to it. After the indentation is made, measuring its depth or width and referring to a set of charts will give a relative hardness number on the scale measured by the particular test method. Notice that, unlike strength, hardness is not a property which allows us to do any further calculations about what forces a substance will resist. Hardness is just a comparative figure.

There are three basic hardness tests whose results might be found in

materials textbooks; BRINELL, ROCKWELL, AND VICKERS The Brinell test uses a round steel ball about 12mm in diameter as

an indenter. In use this indenter is gradually forced into the surface of the test specimen by applying weight gradually, controlled by a hydraulic piston. It makes a circular indentation whose diameter is measured using a small microscope. Looking up a set of tables using the indentation diameter and the test load give s a Brinell Hardness Number, or BHN.

` Because this test makes a fairly large indentation, it is not much use

on thin or small specimens, although it is statistically more valid on large ones. It also cannot test substances which are harder than the hardened steel ball.

The Vickers test uses a similar, but smaller machine, and a much smaller indenter made of industrial diamond, so it can test any material. This indenter is in the shape of a four-sided pyramid, so it leaves a square-shaped dent, whose size is easier to measure. Again, the size of this dent and the test load are used to look up a hardness result, the Vickers Hardness Number, or VHN. The Vickers test is fussy, and harder to use, but it can test any material. It is probably the one most commonly found in dental textbooks to indicating the hardness of a material. The Rockwell test uses a range of different sizes steel balls and diamond cone indenters for material of differing hardness. It measures the depth, not the width of the indentation, and so can give a direct reading of hardness from its dial, without any need for looking up tables. Although it is hard to compare its results with other hardness tests, it is the quickest and easiest to use. Its results

are given, for example, as 55Rc, meaning a hardness of 55 on the Rockwell C scale.

Examples of mechanical and physical properties of various materials In the previous section the properties of polymers, metals and ceramics were discussed in general terms. Now that we know what the particular properties mean, we should consider the values of important properties to show the difference between metals, polymers and ceramics. Melting point The melting point of metal ranges up to that of tungsten at 3410OC, with many of them melting in between 500 and 1500OC. Ceramics have a similar maximum to metals, over 3000OC, but many of the common ones soften above the melting point of most metals. Polymers are generally much lower, only silicone polymers softening at above 4000C Density Metals are the densest materials. The common ones vary from as low as 2.5g/cm3 for aluminium to Lead at 11g/cm3. Gold has a very high density of 19.6g/cm3 and Platinum is the highest at 21.45g/cm3. Chrome alloys have density about 8.5g/cm3. Ceramics have lower densities, from 2.5g/cm3 to 5, and polymers are generally be

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