1. History of the Atomic Theory

3. Democritus - Matter Around 500 BCE
Said everything is made up of matter
He called these matter, atomos (not cut)
This is where the word atom comes from

4. Aristotle - Matter Contrasted Democritus
Said everything is made up of 4 elements
Earth
Fire
Water
Wind
Most believed Aristotle over Democritus for over 2000 years!

5. John Dalton - Atom In 1803, proposed the first atomic theory
Each element are made up of tiny particles called atoms that are solid spheres
Atoms are indivisible and cannot be created nor destroyed
All atoms of each element are identical
All atoms of one element are different from all atoms of another element
Atoms may combine to form compounds
Law of multiple proportions
We call this the “Billiard Ball” model

6. J.J. Thomson - Electron 100 years after Dalton in 1904
His work around a cathode ray tubes
Lead to the idea of negatively charged particles which he called electrons
Thomson said that atoms must be neutral so the rest of the solid sphere must be positive
Called the plum-pudding model

7. Earnest Rutherford – Nucleus and Proton
1911, he did an experiment of bombarding a gold foil with alpha particles
Most alpha particles went through the foil
Some deflected slightly
Some deflected a lot and some even bounced back
He postulated that the atom must have a dense center which is small, he called it the nucleus
He also said the center must be positively charged in which he called protons

8. Niels Bohr – Electron Orbital Model
Around 1913, observed different lines of light produced by excited hydrogen gas
Said that the lines are produced by different electrons in different orbital levels and that each level can only hold a certain amount of electrons

9. Similarities between Rutherford and Bohr
As Bohr was a student of Rutherford and Bohr did his research in Rutherford’s lab, their models are very similar
Typically, their models are blended into one and referred to as the Rutherford-Bohr Model.

10. Homework
Page 142 #4
Page 144 #7 and 9