Chapter 3

The History of the Atom

I. The Scientists and their Discoveries

A.Democritus

1. Date = 400 B.C

2. Discovery = Theorized the smallest unit of an

element

3. Importance = gave the term and definition of atom

Democritus

ATOM

Smallest particle

B. John Dalton1. Date = 18032. Discovery =

a. Every element is made of atoms

b. atoms of same element exactly the same

c. Atoms of different elements join together to form molecules

3. Importance = proposed the ATOMIC THEORY

Atomic Theory =

1. everything is made up of atoms

2. atoms make up molecules

3. atoms of the same element are the same

Molecules made of atoms

John Dalton

Atomic Theory

C. J.J. Thomson

1. Date = 1898

2. Discovery =

a. discovered electrons

b. electrons are negative

3. Importance =

atoms have both negative and positive particles

J.J. Thomson

“Plum Pudding” model

Electrons are negative

D. Ernest Rutherford

1. Date = 1911

2. Discovery =

a. discovered nucleus

b. did a “gold foil” experiment

c. atoms mostly made of space

3. Importance =

a. nucleus is very tiny, and positively charged

b. electrons outside of nucleus

Ernest Rutherford

Gold Foil Experiment

Electrons outside nucleus

Discovered nucleus

E. Niels Bohr

1. Date = 1913

2. Discovery =

a. Electrons are in energy levels

b. s, p, d, f orbitals

c. 1st = 2 electrons;

2nd = 8 electrons;

3rd = 18 electrons

4th = 32 electrons

3. Importance =

electrons orbit around nucleus in a certain pathway

This is the Bohr Model

Niels Bohr

Electrons in energy levels

F. Schrödinger1. Date = present 2. Discovery = electrons are

impossible to find ( a moving fan)

3. Importance =a. Atoms are neutralb. atoms must have the same

number of protons and electrons

Schrödinger

Atoms have no charge

Same number of protons and electrons

II. The Mass of Atom

A. Protons1. are positively charge2. have a mass of 1.67 X 10-27 kg3. Found in the nucleus

B. Neutrons1. Neutral 2. have a mass of 1.67 X 10-27 kg3. Found in the nucleus

C. Electrons

1. are negatively charged

2. Have a mass of 9.11 X 10-31 kg = 0 atom mass

3. Found orbiting outside the nucleus, so their mass does not effect the atom’s mass!

Section Question1. Which scientist gave us the word

atom?2. Which scientist made up the

atomic theory? 3. Every element is made of atoms,

atoms of the same element are exactly the same, different atoms make up molecules. These statements make up the what?

4. Which scientist discovered the electrons?

5. Which scientist discovered the nucleus?

6. Which scientist discovered that electrons where in energy levels?

7. Which scientist said atoms where neutral because they had equal number of protons and electrons?

8. What is the mass of protons?

9. What is the mass of neutrons?

10. What is the mass of electrons?

11. Which part of the atom has the least amount of mass?