The Nature of MatterChapter 1

Inside the Atom

Models of the Atom• Greek philosophers devised a theory of atoms, or tiny

particles• They couldn’t perform experiments and collect evidence like

we do now– They reasoned and formed conclusions

• They reasoned that if you keep cutting matter in half, you would eventually not be able to cut it anymore – They called these particles atoms

John Dalton• He was a school teacher in

England (early 1800s) • Dalton combined the idea

of elements with the Greek theory of the atom

• Matter is made up of atoms• Atoms cannot be divided

into smaller pieces• All atoms of an element

are exactly alike• Different elements are

made of different atoms

William Crookes• Crookes tested Dalton’s theory using

a cathode-ray tube– Almost all air removed– Two pieces of metal called electrodes

(conduct electricity) sealed inside and connected to a battery by wires

– He placed a cross-shaped object between the electrodes

• When he connected the battery, the glass tube lit up with a greenish glow

• On the positive side, there was a shadow of a cross.

• Something was traveling from the negative side to the positive side and was being blocked by the cross

J. J. Thomson• Thomson (1897, England) placed a

magnet by the cathode ray tube and discovered he could move the cathode rays.

• He also used different metals and gasses and discovered that there are negatively charged particles (electrons) in every atom

• Thomson revised Dalton’s model to include a sphere with a positive charge and negatively charged electrons spread evenly within the positive charge

• The negatively charged electrons and the positive charge in the sphere neutralized each other

Earnest Rutherford• Rutherford tested Thomson’s

model in 1906• His team fired fast-moving,

positively charged bits of matter (alpha particles) at a thin film of metal (gold was mainly used)

• If Thomson’s model were true, there isn’t enough charge anywhere in an atom to repel an alpha particle and they would go straight through the thin film of metal– Occasionally on alpha particle would

come directly into contact with a positive charge and be repelled, but not often

Earnest Rutherford – The Reality

• When the alpha particles were fired, many bounced off at large angles

• Rutherford said, “It was about as believable as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.”

• There had a to be a large positive mass somewhere to deflect the alpha particles

• Thus, Thomson’s model could not be correct

A New Atomic Model• The idea of a nucleus was

developed– Rutherford said that almost all

of the mass of an atom must be in the center (nucleus) and must be positively charged

– In 1920, scientists identified the positive charges in the nucleus as protons

• Most of an atom is empty space occupied by nearly massless electrons

• Electrons orbit the nucleus and the number of electrons equals the number of protons in a stable atom

A New Atomic Model• Since electrons have no mass and there are no other particles,

the mass of an atom must equal the number of protons• The mass of atoms, however, is at least twice that of the

number of protons• It was proposed that another particle must be in the nucleus• Later called the neutron and discovered to have almost the

exact mass of a proton and no charge

Electron Shells• This is simplified method of

explaining the orbits of electrons• There are “shells” around the

nucleus kind of like different orbits around a planet.

• Each shell can hold a different amount of electrons:– Shell #1 – 2 electrons– Shell #2 – 8 electrons– Shell #3 – 8 electrons

• Not a realistic explanation of how electrons orbit the nucleus

The Electron Cloud Model

• The electron cloud model explains the unpredictable wave behavior of electrons, which could be anywhere in the area surrounding the nucleus

• These clouds are often referred to as electron orbitals

Orbitals

• There are four different shapes of orbitals corresponding to 4 letters:– s

– p

Orbitals

• d

• f

Orbitals • The orbital closest to the nucleus is the 1s orbital

and it can hold 2 electrons• The 2s is next and can hold two more. Then there

is a 2p orbital that can hold 6 more electrons. • Then comes a 3s (2), 3p (6) and a 3d (10), 4s (2),

4p(6), 4d (10), and 4f (14) • But this method is also screwed up. In actuality,

the orbitals fill up in this order:– 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 5d, 4f, 6p,

7s, 6d, 5f, 7p (or something like that – you don’t need to remember it for sure)

Section 1 Review 1. Name three scientists who contributed to current knowledge

of the atom and explain their contributions. 2. How does the nucleus atom differ from the uniform sphere

model (Thomson’s) of the atom? 3. If a neutral atom has 49 protons, how many electrons does it

have?4. Can you see at atom using just your eye? Is it bigger than

dust? 5. Describe the three kinds of particles found in atoms. Where

are they located in the atom and what are their charges?6. In Rutherford’s experiment, why wouldn’t the electrons in

the atoms of the gold foil affect the paths of the alpha particles?

7. What is an electron cloud?8. How many orbitals are there? What are their names and how

many electrons can they hold?

The Elements• Elements are materials that cannot

be broken down into simpler materials without becoming a different kind of atom

• As of 2002, there were 115 known elements– 90 are naturally occurring – 25 are synthetic elements (made in a

laboratory)

• 4 most common elements in life:– Carbon, Hydrogen, Oxygen, Nitrogen

• 4 most common elements in the Earth– Oxygen, silica, aluminum, iron

The Periodic Table

• The Periodic Table is a chart that organizes and displays information about the elements

• The elements represented by the symbols on the periodic table are placed purposely in their position on the table

• Even the shape of the periodic table is that way on purpose

The Periodic Table

• The rows are called periods– They have the same number of energy levels (1

in the 1s, etc.)

• The columns are called groups– They have similar properties and tend to form

similar bonds

Atomic Number• The atomic number is the top number in the

element’s periodic table block• Tells the number of protons in the nucleus of each

atom of an element– Also the number of electrons in an electronically neutral

atom

• The number of protons remains constant in every atom of an element

Isotopes and Ions• Isotopes are atoms of the

same element that have different numbers of neutrons

• Ions are atoms of the same element that have different numbers of electrons– Except for the elements in

Group 18, all atoms have an empty spot in their outermost electron orbital and “nature” hates this

– So atoms either lose or gain electrons to fill these spots

Mass Number and Atomic Mass• Mass number is the number of protons plus the number of

neutrons (different mass numbers for different isotopes)• Atomic mass is the average mass of all the isotopes of an

element• The atomic mass is found below the element symbol• The unit used for atomic mass is the atomic mass unit, or “u”

Metals• The majority of elements

are metals (blue in your book)

• Metals have a shiny luster• Good conductors of heat

and electricity• Solids at room

temperature• Malleable (can be shaped)• Ductile (can be drawn

into wires without breaking)

Nonmetals• Nonmetals are found on the

right side of the periodic table (yellow in your book)

• Dull luster• Poor conductors of heat and

electricity• Many are gases at room

temperature• Brittle (cannot change shape

without breaking)• 96% of the human body is

made up of nonmatals

Metalloids• Metalloids are found

between the metals and nonmetals on the periodic table (green in your book)

• Have characteristics of both metals and nonmetals

• Do not conduct heat and electricity as well as metals

• All are solids at room temperature

Section 2 Review1. What is an element?2. What are the four most common elements in Earth?3. Describe the difference between atomic number and atomic

mass.4. What are isotopes? What is different in each isotope?5. Explain some uses of metals. 6. What are the three different kinds of elements? 7. What is an isotope?8. What is an ion?9. Hector is new to your class today. He missed the lesson on

how to use the periodic table to find information about the elements. Describe how you would teach Hector to find the atomic number for the element oxygen. Explain what this information tells him about oxygen.

Substances

• Matter that has the same composition and properties throughout is known as a substance.

Compounds• A compound is a substance whose

smallest unit is made up of atoms of more than one element

• Written in formulas– The subscript number tells how many

atoms of the preceding element are in the compound

• Ex: CO2

– No subscript is used when only one atom of the element is present

• Ex: HCl

• A given compound is always made of the same elements in the same proportion– Ex: Water is always H2O, never HO2

or H3O

Mixtures• Mixtures occur when two or more substances mixed

together which don’t make a new substance• Unlike compounds, the proportions of the substances can

be changed• Examples: air, blood• Mixtures can be separated easily• Homogeneous mixtures are the same throughout the whole

mixture– Ex: Kool-aid, soup broth

• In heterogeneous mixtures, you can see the different parts– Ex: most rocks, tacos

Section 3 Review

1. List three examples of compounds and three examples of mixtures. Explain your choices.

2. How can you tell that a substance is a compound by looking at its formula?

3. Which kind of mixture is sometimes difficult to distinguish from a compound? Why?

4. What is the difference between homogeneous and heterogeneous mixtures?

5. Was your breakfast a compound, a homogeneous mixture, or a heterogeneous mixture? Explain.