Atomic Theory

The Who’s Who of Atoms

Democritus

• ~460 BCE.• Defined atom as the “smallest

bit of matter.• 100 years later, Aristotle

dismissed his idea as “worthless.”

John Dalton

• 1805• Said matter consisted of

elementary lumpy particles• First Atomic Theory• 5 Parts to his model:

Dalton’s Atomic Model

1. All matter is made of tiny particles called atoms.

2. All atoms of one element are alike.3. Atoms of one element are different

from atoms of other elements.4. Compounds are formed when atoms

from different elements combine.5. Chemical reactions involve re-

arrangement of atoms, not changes in atoms themselves.

J.J. Thomson

• 1897• Discovered the electron with his

Cathode Ray Tube Experiment• Came up with the “Plum Pudding

Model” of atom

Cathode Ray Tube Experiment

Plum Pudding Model

“Real” Plum Pudding

Max Planck

• 1900• When you vibrate atoms enough

(heat it until it glows), you only get energy in whole intervals: energy packets called “quanta.”

Ernest Rutherford

• 1911• Discovered the nucleus with his

Gold Foil Experiment•Nucleus: the small, dense, positively-

charged center of the atom

• In 1919, he identified “protons”

Rutherford’s Atom

Robert A. Millikan

• 1916• Found charge and mass of

electron with his Oil Drop Experiment

Oil Drop Experiment

Niels Bohr

• 1912• Developed planetary model of

the atom• Discovered that electrons move

around the nucleus at set distances

• Called these “electron orbitals.”

Electron Orbitals

“Bohr Model” of the Atom

James Chadwick

• 1932• Discovered the neutron

So Then We Have This!

Let’s Memorize These Guys!

• It’s the least we can do to commemorate all their hard work!

• Quiz on Friday!

• 5 minute break? Then SUBATOMIC PARTICLES!