1. Atomic Models From the ancient Greeks to the 20 th century

2. Democritus’s “model” Democritus only proposed the philosophical idea of atoms

3. Dalton’s Model Dalton proposed the first Atomic Theory – All matter is composed of tiny particles – atoms – Atoms of a given element are all identical to each other – Atoms cannot be divided, created or destroyed – Atoms combine in simple whole # ratios to form compounds

4. JJ Thompson Proposed the “Plum Pudding” Model of atoms – The majority of an atom was a mass of positive charge with negatives (electrons) imbedded throughout

5. Rutherford’s Nuclear Model Based on his famous gold foil experiment, Rutherford found evidence that atoms have a tiny region of positive charge (nucleus), surrounded by negative charge

6. Bohr’s Model Neils Bohr proposed that electrons are found in distinct energy levels that are a certain distance from the nucleus

7. The Modern Quantum Mechanical Model Based on work done by Louis deBroglie, Schroedinger proposed the wave mechanical (aka electron cloud) model of the atom, where electrons are likely to be found in “orbitals” of different shape.

8. Other key scientists involved… Antoine Lavoisier – developed the law of conservation of mass Robert Millikan – experimentally determined the mass and charge of an electron Henry Moseley – determine the number of protons in the nucleus of several atoms James Chadwick – discovered neutrons Max Planck – developed the idea that energy is quantized (exists in bundles or packets) Werner Heisenberg – developed the Uncertainty Principle – one cannot know the exact position and speed of an electron simutaneously