3 physci myp atomic structure

M4 M4 PhySciPhySci Matter III: Matter III: Atomic StructureAtomic Structure

M4 physci Matter: Atomic Structure Slide 2 of 46

Learning ObjectivesLearning Objectives

• Concepts

– Microscopic, macroscopic, sub-microscopic, atom, molecule, ion, group, period

– Atomic structure, electron, proton, neutron, electron configuration, shell, outer shell, valence shell, valence electron, core shell, core electrons

– Monatomic (atomic) element, diatomic element, molecular element

– Law of conservation of matter, law of constant composition (proportion),

• Skills

– State that all matter is made up of atoms (atomic theory) and explain the structure of an atom.

– Electrical forces between oppositely charged electrons and positive nucleus (protons) hold atoms together.



– Explain that the enormous variety of materials in the world is the result of different combinations of a relatively small number (approximately 100) of basic ingredients (elements).

– State that the smallest part of an element (macroscopic) is an atom (sub-microscopic).

– Explain the law of constant proportions and how it relates to the submicroscopic and macroscopic views of matter.

– Explain that the macroscopic world is made up of sub-microscopic particles (atomic theory) and give several examples of evidence.

– State that the number of positive protons in the nucleus determines the identity of the atom.



– Be able to recall the symbol, position (atomic number), electronic structure and electronic configuration for the first 20 elements in the periodic table.

– Explain the significance of the noble gas electronic structures and valence electrons.


Law of Conservation of MassLaw of Conservation of Mass

• The law of conservation of matter states that matter is neither created nor destroyed in any process.

During any change (physical or chemical) matter cannot be created or destroyed.

This can be easily demonstrated by conducting a reaction in a closed container placed on a top pan balance.

The mass of the reactants before and after the reaction will remain constant as long as nothing escapes into the atmosphere from the reaction container.

M4 Physci Matter: Atomic Structure Slide 6 of 46

How does this support the contention that How does this support the contention that matter is made up of atoms?matter is made up of atoms?

• It does because if matter were made of indivisible atoms then the atoms that you start with must be the same as the atoms that you end with, but the way in which the atoms are arranged can change.

Physical change: the atoms AND molecules in the starting substances are the same as the atoms AND molecules in the ending substances.

– The molecules may just be moving at a different speed or located different distances apart (see Chemistry Matter: State and Energy).

• Chemical change: the atoms are the same in the starting substances and the ending substances, BUT the molecules are different.

– The molecules have hit each other, broken apart, and then rearranged and bonded into new molecules.

– These new molecules have different properties than the starting substances.


Law of Constant ProportionLaw of Constant Proportion

For instance mass of oxygen in water is always 88.79% oxygen and 11.21% hydrogen.

Mass of carbon in carbon dioxide is always 27.29% carbon and 72.71% oxygen.

How does this support Democritus’ hypothesis about what matter is made up of?


Dalton’s Atomic Theory of MatterDalton’s Atomic Theory of Matter

• In 1803 John Dalton, and English schoolteacher, studied these other findings and formulated the first comprehensive atomic theory of matter.

His theory were based on the following postulates:

– Each element is composed of extremely small particles called atoms.

– Atoms of a given element are identical but they differ from those of any other element.

– atoms are neither created nor destroyed in any chemical reaction.

– A given compound always has the same relative number and kinds of atoms.

• As you learn more chemistry, you will discover that there are small but important exceptions to some of Dalton’s postulates.


AtomAtom

• Nowadays, an atom is defined as the smallest particle of an element that retains the chemical identity of that element.

There are 92 natural elements, which means there are 92 different naturally occurring atoms as well.

– The rest of the elements appearing on a modern periodic table are synthetic.

M4 Chemistry Matter: Atomic Structure Slide 10 of 46


Atoms…aren’t the smallest particlesAtoms…aren’t the smallest particles

• Important discoveries have been made since Democritus and Dalton to warrant a revision of the way we look at atoms of course.

For instance, we now know that atoms aren’t the smallest particles, contrary to what Democritus and Dalton proposed.

Atoms are composed of protons, neutrons and electrons.

– Protons and neutrons are found in the center of the atom, called the nucleus.

– Electrons are found in shells orbiting around the nucleus not very much unlike planets around the sun.


NucleusNucleus

• The nucleus is a tiny and extremely dense region in the centre of the atom and most of the rest of the atom is just empty.

– If a pea were placed in the center of a football stadium, it would represent the nucleus to the atom of the stadium!!

• Because they are found in the nucleus of the atom, protons and neutrons are often referred to as nucleons.

Electrons move around the nucleus in orbits in much the same way as the planets orbit the sun in the solar system.


Electrostatic AttractionElectrostatic Attraction

• Do you remember which sub-atomic particles have which charge?

Protons have a POSITIVE charge.

Electrons have a NEGATIVE charge.

Neutrons are NEUTRAL in charge.

Electrons are kept in orbit by the ELECTROSTATIC ATTRACTION between the negative electrons and the positive protons.

Electrons are kept in orbit by electrostatic attraction


Properties of sub-atomic ParticlesProperties of sub-atomic Particles

Outside nucleus, Outside nucleus, orbiting nucleusorbiting nucleus

-1-11/1840 1/1840 (negligible)(negligible)

ElectronElectron

in nucleusin nucleus0011NeutronNeutron

in nucleusin nucleus+1+111ProtonProton

Location in atomLocation in atomRelative chargeRelative chargeRelative massRelative massParticleParticle


Neutrality of AtomsNeutrality of Atoms

• Atoms are electrically neutral; they always contain the same number of positively charged protons and negatively charged electrons.

For example, an atom of magnesium contains 12 positively charged protons in the nucleus and 12 negatively charged electrons orbiting the nucleus.

Charge on an atom of magnesium

= charge on protons + charge on electrons

= 12 + (−12)

= 0

Because atoms contain the same amount of positive and negative charge they are electrically neutral.


IonIon

• If an atom gains or loses electrons then the balance is upset and the atom becomes electrically charged.

Atoms which have become electrically charged by gaining or losing electrons are given a different name: they are called ions (about which we will have more to say later on in the course).

Next we are going to look at the arrangement of these electrons around the nucleus of different atoms.

We will look at the first ten elements in the periodic table.


Atomic Structure: HydrogenAtomic Structure: Hydrogen

• Hydrogen is the simplest element.

It has one electron and that electron in hydrogen atom is found in…


HydrogenHydrogen

• …the first shell.

A shell is like an orbit, like the orbit of the earth around the sun where the electron is said to be going around the nucleus.

The diagram above showing the electron in the first shell of hydrogen is referred to as the electronic structure of hydrogen.


HeliumHelium

• The next element in the periodic table is Helium and it contains 2 electrons.

The second electron in Helium is found orbiting the nucleus in…


HeliumHelium

• …the first shell as well.

The electronic structure of helium therefore contains two electrons I the outer shell.

1H2He


HeliumHelium

• The first shell in Helium has as many electrons as it can take.

That’s why helium is referred to as a noble gas, an inert (unreactive) gas.

It has a full outer shell of electrons and is therefore energetically stable.

1H2He

M4 Physc Matter: Atomic Structure Slide 22 of 46

LithiumLithium

• With the next element, Lithium, we move into the second period and so the third electron in this element is found orbiting in the second shell.

3Li


LithiumLithium

• A period is a row of elements on the periodic table.

Remember again that the first shell can accommodate a maximum of 2 electrons.

3Li


BerylliumBeryllium

• As we move from one element to the next in this period, the extra electron is found in the same shell as Lithium, the second shell.

3Li4Be


BoronBoron

3Li4Be 5B


CarbonCarbon

3Li4Be 5B 6C


NitrogenNitrogen

3Li4Be 5B 6C

7N


OxygenOxygen

3Li4Be 5B 6C

7N 8O


FluorineFluorine

9F

3Li4Be 5B 6C

7N 8O


NeonNeon

• And with neon we come to the last element in this period and the maximum number of electrons (8) the second shell in any element can hold.

10Ne9F

3Li4Be 5B 6C

7N 8O


Valence Shell & Valence ElectronsValence Shell & Valence Electrons

• The outermost shell where electrons are found for a given element is referred to as the valence shell.

– For example in the case of hydrogen and helium the valence shell is the first shell.

– For those in the second period (from lithium to neon) the valence shell is the second shell.

– Those in the third period the valence shell is the third shell etc.

• The electrons found in the valence shell are therefore referred to as valence electrons.

– Hydrogen has 1 valence electron (and some times it’s referred to as having a valency of 1)

– Helium 2, lithium 1, boron 3, nitrogen 5 etc.


Electron configurationElectron configuration

• The arrangement of electrons, the electron configuration, in shells can also be indicated by a sequence of numbers representing electrons in each shell starting with the inner most core shell.

H – 1 He – 2

Li – 2, 1 Be – 2, 2 B – 2, 3 C – 2, 4

N – 2, 5 O – 2, 6 F – 2, 7 Ne – 2, 8

Na – 2, 8, 1 Mg – 2, 8, 2 etc.


Exercise: Electronic Configuration of Group 1 MetalsExercise: Electronic Configuration of Group 1 Metals

A column of elements are referred to as a group.

Use the information shown to write down the electronic configurations of the first three group one metals and then draw the electronic configuration for lithium.

2,1

2,8,1

2,8,8,1

Lithium

M4 PhysciMatter: Atomic Structure Slide 34 of 46

Electron ConfigurationElectron Configuration

• For the first 20 elements, the following guidelines can be used

• The first shell can hold a maximum of 2 electrons

• The second shell can hold a maximum of 8 electrons

• The third shell can hold a maximum of 8 electrons

• Any remaining electrons can be placed in the fourth shell


ConclusionsConclusions

• Notice then that the number of shells the atom of an element contains is the same as the period (row) in which the element appears in the periodic table.

– hydrogen (H) and helium (He) for example appear in period 1 and contain only 1 shell each

– Lithium (Li) and the rest of the elements in period 2 contain two shells each etc.

• The number of electrons in the valence shell (outermost shell) is the same as the group (column) to which the element belong.

– Lithium (Li), Sodium (Na) and the rest of the elements below in Group 1 contain 1 electron in their valence shell

– All elements in the last column (group 8) (with the exception of helium) contain 8 electrons in the valence shell etc.


Practice QuestionsPractice Questions

• 1. J05/2/5f. Astatine, At, is below iodine in Group VII of the Periodic Table.

(i) How many protons does astatine have in its nucleus? [1]

(iii) The most common isotope of astatine has a nucleon number (mass number) of 210. Calculate the number of neutrons in this isotope of astatine. [1]



• 2. J04/2/5. Look at the list of five elements below. argon bromine chlorine iodine potassium

(a) Put these five elements in order of increasing proton number. [1]

(b) Put these five elements in order of increasing relative atomic mass. [1]

(c) The orders of proton number and relative atomic mass for these five elements are different. Which one of the following is the most likely explanation for this? Tick one box.

The number of protons must always equal the number of neutrons.

The atoms easily gain or lose electrons.

The presence of neutrons.

The proton number of a particular element may vary.



• 3. The diagram represents an atom. Choose words from the list to label the diagram.

electron ion neutron nucleus

p r o t o n



• 4. This question is about the elements in the list below.

aluminium astatine chlorine fluorine

francium helium hydrogen iodine

iron potassium

Which element, from the list above:

(a) is in Group 3 of the Periodic Table, ...........................................

(b) is in Period 5 of the Periodic Table, ...........................................



• 5. Use a periodic table to help you answer this question. This question is about elements and atoms.

(a) About how many different elements are found on Earth?Draw a ring around the correct number.

40 50 60 70 80 90 [1]

(b) The following are parts of an atom: [3]

electron neutron nucleus proton

Choose from the list the one which:

(i) has no electrical charge;

(ii) contains two of the other particles;

(iii) has very little (negligible) mass.


Practice Questions: Multiple ChoicePractice Questions: Multiple Choice

• 1. Which one of the following is not one of the postulates of Dalton's atomic theory?

A) Each element is composed of tiny, indivisible particles called atoms.

B) All atoms of a given element are identical to each other and different from those of other elements.

C) During a chemical reaction, atoms are changed into atoms of different elements.

D) Compounds are formed when atoms of different elements combine.

E) Atoms of an element are not changed into different types of atoms by chemical reactions.

• 2. A molecule of water contains hydrogen and oxygen in a 1:8 ratio by mass. This is a statement of __________.

A) the law of multiple proportions B) the law of constant composition

C) the law of conservation of mass D) the law of conservation of energy

E) none of the above



• 3. Which one of the following is not one of the postulates of Dalton's atomic theory?

A) Atoms are composed of protons, neutrons, and electrons.

B) All atoms of a given element are identical; the atoms of different elements are different and have different properties.

C) Atoms of an element are not changed into different types of atoms by chemical reactions: atoms are neither created nor destroyed in chemical reactions.

D) Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

E) Each element is composed of extremely small particles called atoms.



• 4. Of the following, the smallest and lightest subatomic particle is the __________.

A) Neutron B) proton C) electron D) nucleus E) alpha particle

• 5. All atoms of a given element have the same __________.

A) mass B) number of protons C) number of neutrons

D) number of electrons and neutrons E) density

• 6. The atomic number indicates __________.

A) the number of neutrons in a nucleus

B) the total number of neutrons and protons in a nucleus

C) the number of protons or electrons in a neutral atom

D) the number of atoms in 1 g of an element

E) the number of different isotopes of an element

3 physci myp atomic structure

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