1. History of Atomic Theory

2. Atomic Theory Timeline

3. Democritus: B.C.
• 1. There are basic elements from which all matter is made • 2. Everything is made of small atoms moving in a void • 3. Some atoms are round, pointy, oily, have hooks, etc. to account for their properties • 4. Ideas rejected by leading philosophers

4. John Dalton:
• 1. Each element is composed of extremely small indivisible particles called atoms • 2. All the atoms of a given element are identical, but different from those of any other element • 3. Atoms are neither created nor destroyed in any chemical reaction • 4. A given compound always has the same relative numbers and kinds of Atoms. (Ex water is always H2O)

5. John Dalton:
• Where Dalton was wrong – 1. We can divide the atom • Subatomic particles include protons, neutrons, & electrons • E = mc2  Atomic Bomb – 2. Not all atoms of a given element are identical • Isotopes

6. J.J. Thomson:
• 1.Discovered electron in 1897 – Cathode Ray Tube Experiment – Electron has (-) charge: “e-” – Mass of e- = 9.11 x 10-28g • 1/1840 the mass of a proton – Protons discovered in 1886 by Goldstein • Proton has (+) charge: “p+” • Mass of p+ = 1.67 x 10-24g

7. J.J. Thomson:
• Cathode Ray Tube Experiment • The beam Is attracted to (+) plate and repelled by (-) plate so it must be made of (-) charged particles!

8. J.J. Thomson:
• Thomson knew the atom as a whole was neutral so there must be (+) charged particles also.

9. J.J. Thomson:
• 2. Developed Plum Pudding model in 1904 – Atom is a sphere of (+) charge with (-)electrons randomly scattered throughout – Modern version: “Choc. Chip Cookie” ?

10. J.J. Thomson:
• 3. Discovered isotopes in 1913 – Different “versions” of the same atom. Atoms of the same element, but with different masses. • Nucleus and neutrons had not been discovered yet so he didn’t fully understand why isotopes existed

11. Ernest Rutherford:
• 1. Discovered Nucleus w/“Gold Foil Experiment” in 1909 – Bombarded a piece of gold foil (gold atoms) w/large, (+) charged alpha particles. – Most passed right through but a few bounced back or were deflected at an angle

12. Ernest Rutherford:
• 2. Developed Nuclear Atom Theory in 1910 – 1. The atom is mostly empty space – 2. In the center of the atom is a tiny nucleus, which contains most of the atom’s mass – 3. The nucleus has a (+) charge – 4. Electrons are in afixed orbit in the empty space around the nucleus

13. Ernest Rutherford:

14. James Chadwick:
• Discovered the neutron in 1932 – Neutron has no charge: “n0” – Mass of n0: 1.67 x 10-24g • Almost the same mass as a proton (both are much larger than the e-) • Described the nucleus and explained isotopes discovered by JJ Thomson in 1913 – Isotopes of the same element have different masses (same # protons but different #s of neutrons)

15. Niels Bohr: 1885-1962 • Developed Planetary Model in 1913
Nucleus is surrounded by electrons that are orbiting in definite paths
“like planets orbit the sun”

16. Niels Bohr:
2. Electrons exist in discrete “quantized” energy levels.
• Quantized = the opposite of continuous, goes from one level to the next with nothing in between