What are important discoveries related to the structure of an atom?

• Three sub-atomic particles:

• Atom – basic unit of matter

• Element –substance consisting of one type of atom

• Compound – substance consisting of two or more different atoms

Democritus The Laughing Philosopher

- Matter composed of atoms (tiny indestructible units)

- No experiments, Little evidence

- Smooth (sweet) and sharp (sour) atoms

John Dalton 1808Dalton's Atomic Theory

1) Matter is made of atoms which are indivisible and indestructible.

2) All atoms of an element are identical in mass and properties.

3) Compounds are formed by two or more different kinds of atoms coming together.

4) A chemical reaction is a rearrangement of atoms.

JJ Thomson 1897• Lots of positive space

with negative clusters

• Cathode Ray Experiment

• Discovered electrons

• Plum Pudding Model

• Chocolate Chip Muffin

What is plum pudding?

Ernest Rutherford 1908

• Atoms= mostly empty space

• Discovered nucleus that contained positively charged particles

• Gold Foil Experiment

• Particles were deflected

• Electrons circle nucleus

Robert Millikan 1909

• Oil Drop Experiment

• Calculated mass and charge amount of electron

Flea on a Elephant

Neglible Zero Mass

Neils Bohr 1913• Electrons move in orbitals

(energy levels) around nucleus• Like planets and the sun • Each orbital holds a certain

number of electrons

James Chadwick 1932

• Discovered neutrons

• Neutral particle with mass close to proton

Current Model

• Watch “Atomos, Atomos” Video

• Watch Rutherford's Gold Foil Experiment