October 13, 2010: Atomic Theory (Angelo)

Atomic Theory:
- Many theories have been made to explain atoms. Not all of them are true today.

Aristotle (384 B.C. to 322 B.C.):
- Invented the four elements theory. (Water, Earth, Wind, and Fire)
- The four elements theory lasted for about 2000 years.
- It is not a scientific theory because it could not be tested against observation.

Democritus (460 B.C. to 370 B.C.):
- In 300 B.C., Democritus said atoms were indivisible particles.
- This was the first mention of atoms (atomus).
- Not a testable theory, only a conceptual model.
- No mention of any atomic nucleus or its consituents.
- Cannot be used to explain chemical reactions.

Lavoisier (1743 to 1794):
- Created Law of Conservation of Mass.
          - States that the mass of a system will remain constant.
- Created Law of Definite Proportions
          - Water is always 11% H (Hydrogen) and 89% O (Oxygen)

Proust (1754 to 1826):
- If a compound is broken down into its constituents, the products exist in the same ratio as in the compound.
- Proust experimentally proved Lavoisier's laws.

Dalton (1766 to 1844):
- Thought atoms are solid, indestructable spheres (like Billiard balls).
- Thought each element had different types of atoms (different color, shape, etc.).
- Based on the Law of Conservation of Mass.
- Have a molecule (atoms combine in simple whole number ratios) explains the Law of Constant Composition.
- If the atoms are not destroyed then the mass does not change.

J.J. Thompson (1856 to 1940):
- Raisin bun
- Solid, positive spheres, with negative particles embedded in them.
- First atomic theory to have positive (protons) and negative (electron) charges.
- Demonstrated the existence of all electrons using a cathode ray tube

Rutherford (1871 to 1937):
- Showed that atoms have a positive, dense centre with electrons outside it.
- Resulted in planetary model.
- Explains why electrons spin around nulceus.
- Suggest atoms are mostly empty space.