CHAPTER 4:

Matter is Made up of Atoms

ATOMS & THEIR STRUCTURE

Aristotle thought matter was

made of air, earth, fire and

water.

Democritus (250 B.C.)-

Said the world is made of empty space

& tiny particles called atoms

DEVELOPMENT OF THE ATOMIC THEORY

(3 SCIENTISTS INVOLVED)

1. Lavoisier (1743-1794)-

Law of Conservation of Matter

The Father of Modern Chemistry

Law of definite proportions-elements that make up compounds are found in fixed proportions

2. PROUST (1799)

Joseph Proust

3. DALTON’S ATOMIC THEORY

1. All matter is made of

atoms.

2. Atoms are indivisible

3. All atoms of 1 element 3. All atoms of 1 element

are alike, but are

different than atoms of

other elements

Was he right?

1. All matter is made of atoms.

2. Atoms are indivisible

3. All atoms of 1 element 3. All atoms of 1 element are alike, but are different than atoms of other elements

DISCOVERY OF ATOMIC STRUCTURE

4 SCIENTISTSJJ Thomson (1897)

1. Discovered the electron during his vacuum

tube/cathode ray experiments.

2. JJ’s Plum Pudding model

3. When studying Ne he discovered

isotopes-

atoms of an element that are alike chemically,

but differ in mass.

*Same # protons, different # neutrons

4. His work led to the discovery of the neutron

NAGAOKA

Saturnian model or

early planetary

model

RUTHERFORD

A. Gold foil experiment

1. shot positive particles through a

thin sheet of Au.

2. Most went straight through, but

others were deflected.others were deflected.

WHY?

http://www-outreach.phy.cam.ac.uk/camphy/nucleus/nucleus6_1.htm

CONCLUSIONS DRAWN FROM THE GOLD

FOIL EXPERIMENT

1. Most of the atom is empty space.

2. Discovery of the nucleus- small,

dense, positively charged central

core of the atomcore of the atom

NEILS BOHR (1913)1. Given credit for the planetary model of the atom.

2. Was more specific than Nagaoka because he Nagaoka because he concluded that electrons are arranged in E levels.

BOHR’S STATEMENTS ABOUT

ELECTRONS

1. Said that electrons absorb E &

move to higher states.

2. These “excited” electrons give

off that E as light waves & fall off that E as light waves & fall

back to a lower E level.

Neils Bohr’s Atomic Model

An excited electron jumps

to an outer E level.

It falls back, releasing light It falls back, releasing light

waves.

The “packet of light”

released is called a

photon.

+

Nucleus

Electron

Orbit

The atom is not flat!

Orbit

Energy Levels

Our View of Electrons Today

�E levels are not neat orbits

�E levels are spaces around the

nucleus where electrons are

most likely to be foundmost likely to be found

DaltonNagaokaJJ Thomson

Rutherford

BohrToday’s model

Subatomic particle Mass in µ Charge

Protons 1.01 Positive

Neutrons 1.01 Neutral

Subatomic Particles

Mass of atoms is measured in atomic mass units (µ, or amu)

Neutrons 1.01 Neutral

Electrons 0.0005 Negative

Determining Atomic Structure

1. Atomic number-

the # of protons in the nucleus & the #

of electrons if the atom is neutral

2. Atomic mass/Mass number-2. Atomic mass/Mass number-

the sum of protons + neutrons in the

nucleus

Carbon12.011

C6

Atomic number

Atomic mass

Atomic number

# protons = 6

# electrons = 6

# neutrons = 12-6= 6

You try itHow many protons, neutrons, and

electrons are present in nitrogen?

p = 7

e = 7

n = 14-7=7n = 14-7=7

Try bromine

p = 35

e = 35

n = 45

Why isn’t atomic mass a whole

number?

� It is a weighted average of all isotopes of an

element.

� The isotope most common in nature is the one

that has a mass closest to the one found on the

periodic table.periodic table.

Ex:

Most C has a mass of 12.

The most common isotope of

carbon is therefore C-12

Electrons in Energy Levels

�1st E level can hold a max of 2

electrons

�2nd E level can hold 8

�3rd can hold 18

�4th can hold 32

Drawing atoms

Draw an atom of beryllium

#p= 4

#n=9-4=5

#e=4

# valence electrons = 2

Beryllium9.012

Be4

Valence electrons = #

electrons in outermost E

level

Try to draw an atom of phosporus on

your own

P=15

Valence = 5

P=15

N=16

This is P-31.

P=15P=15

N=16

This is P-33.

P=15

The only thing

different is the

# of neutronsP=15

N=18

Drawing ions

P=15

To draw a +1

ion, take away

1 electron

This is a neutral

atom of P-31

Now the P=15

N=16 atom has a

+1 charge

Drawing ions

P=15

To draw a -1

ion, add 1

electron

Now the P=15

N=16 atom has a

-1 charge

Drawing ions

P=15

What do you do

to make the

atom have a -3

charge?

P=15

N=16

IN SUMMARY

You have learned about:

�The development of the atomic

theory

� The discovery of atomic structure� The discovery of atomic structure

� How to construct a model of an

atom of any element using the

periodic table

The End!!!!!!!!!