I. Theory of the Atom

A. Ancient Greece (2000+ years ago)

Democritus Believed that matter could not be continuously divided

Matter consists of small indivisible particles

“Atom” = indestructible

Particle are in continuous motion

Four elements make up all matter and energy

Earth, Earth, Wind, Wind, Water, Water, Fire, Fire,

No scientific evidence to show this.No scientific evidence to show this.

B. Dalton’s Atomic Theory - 1803

John Dalton proposed the theory that:1. All matter is composed of small particles which cannot be broken down (atoms)

2. All atoms of the same element are identical in size, mass and properties. Atoms of different elements are different in size, mass and properties3.Atoms of different element combine in simple ratios to make compounds

4. In chemical reactions, atoms are combined, separated, or rearranged (No atoms are created or destroyed)

H2S PbO2

So at this point we believe that an atom is like a small solid ball of matter that cannot be split up

C. J.J. Thompson

Experimented with cathode rays

Used electric field to show that cathode rays are negatively charged particlesAll tested substances gave off the same negative particles

Matter must be made of smaller particles, including the negative one

After discovery of the proton, Thompson assumed an atom was a mixture of + and – charged particles, all mixed up

Plum-Pudding Model

+ - + -- + -

- + - +

D. Rutherford

Ernest Rutherford wanted to determine what an atom looked like.

Fired + charged alpha rays at a very thin piece of gold (Gold foil)

Put a screen behind the foil to determine what happens to the rays

Radioactive sourceGold foil

Screen

Most of the rays went straight through the foil

Some rays deflected

A very few rays came straight back

- - - - - -

Rutherford explained this by stating that

a. Atoms have mostly empty spaceThis is why most rays went straight through

b. There must be a dense positive center to an atom

This is why the positive rays deflected. (Positive rays are repelled by positive charged objects)

Later experiments showed that electrons exist in the space between nuclei

+

Rutherford model

e- e-

E. Bohr Neils Bohr looked at the arrangement of electronsNeils Bohr looked at the arrangement of electrons

Electrons exist in definite areas around the nucleus

Further from the nucleus, an electron had more energy

Electrons can gain energy and “jump” to higher levelsThey can then give off the energy as they jump back downBohr named the levels K,L,M,N,O… with K begin closest to the nucleus

Now renamed 1,2,3,4… with 1 being closest to the nucleus and having the least amount of energy

All atoms have the same types of energy levels

Energy levels

nucleus

e-e- e-

M L K

3 2 1

+

- - -

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Planetary model

F. Wave Mechanical Model

Modified Bohr’s model

Electrons are not in circular orbits, but exist in specific spaces around the nucleus

Pattern is random, unpredictable

level level

Energy levels contain sublevels

nucleus

Dr. Wave Mechanical