History of the Atom

John Dalton

British Schoolteacher, 1808 First to revisit the “atom” and support his

theory experimentally Atomic Theory

1. Atom-indivisible solid sphere2. All atoms of same element have same mass and

chemical behavior3. Atoms of different elements always combine in

fixed number ratios to produce specific compounds.

William Crooke

Developed the Crooke’s tube AKA Cathode Ray Tube * Used to determine the nature of the rays

Shadow (light or electric charge)

JJ Thomson

The Experiment that proved Light or Charge**

Particles positively or negatively charged? Used charged plates to test the direction of the

cathode ray. *** The rays repelled the negative plate.

What does that mean?

JJ’s Model of the Atom

The particles are negative! Called Electrons. Further experiments using Hydrogen gas

lead to discovery of Protons Model

Ernest Rutherford

New Zealand Chemist, 1909

Famous Gold Foil Experiment—Testing Thomson’s model of the atom***

movie

Details

Alpha Particle 2 protons & 2 neutrons 4 times the mass of

Hydrogen Overall positively charged

Results There’s something small

and dense in the center

James Chadwick

Experiment Alpha source shoots alpha rays at Beryllium

(Beryllium - Be – atomic number = 4) Beryllium rays are shot at wax Wax ray is formed Charged plates are presented and wax ray gets

deflected toward negative plate

Chadwick

Why didn’t the Beryllium ray get deflected!? The ray was neutral and unaffected by the charged plates

Why was the wax ray deflected? Because it was repelled by the positive plate (it must

have carried a positive charge) Why was a wax ray created in the first place?

Elastic collisions – total kinetic energy is conserved This also means protons and neutrons have the same

mass

Chemistry Humor

A neutron walks into a bar and asks the bartender “how much for a drink?”

The bartender replies, “for you, NO CHARGE”