1. Dalton’s Atomic Theory
Still accepted except
Atoms are indivisible

2. J.J. Thomsom
1897
Discovered
negatively charged subatomic particle- the electron
By mass of electron
1 / 2000 mass of H atom

4. Cathode Rays Negatively charged particles moving at high speeds
Same regardless of type of metal or type of gas used
Therefore, electrons are part of all atoms

5. Robert Millikan Determined charge of electron
Determined the charge to mass ratio of electron
1916- calculated mass of electron
Electron → charge -1 →
mass 1 / 1840 of H atom

6. Atom? Atoms are electrically neutral
Electrical charges are carried by matter
Electric charges are all in whole number multiples of a basic unit
When positive charges = negative charges the result is neutral

7. E. Goldstein
1886-Found rays in cathode ray tube moving opposite the cathode rays
Called them- canal rays
Made of Positive particles
Called Protons
Mass 1840 times mass of electron

8. James Chadwick 1932 Found another fundamental particle No charge
Mass about equal to proton
Called- Neutron

10. Ernest Rutherford
Explored atom with alpha particles (helium atoms with +2 charge- lost 2 electrons)
Directed alpha particles into thin layer of gold foil
Prediction: alpha particles would pass through foil with very little deflection

12. Results Most alpha particles passed straight through foil
Few particles bounced back at large angles toward alpha particle source

13. Rutherford’s New Atom 1.The atom is mostly empty space
This accounts for most of the alpha particles passing straight through the foil

14. 2. Positive charges and most of the mass are concentrated in a small region.
This accounted for the large angles of deflection.

15. Nucleus Named by Rutherford Positive, massive region in center of atom
Made of protons & neutrons

16. Comparison If Lancer stadium was an atom:
Then the nucleus would be the size of a marble at the 50 yard line!