1. DEMOCRITUS (460-370B.C.) Atoms are indivisible, smallest particles

2. JOHN DALTON (1805) Atoms are smallest, permanent

3. J.J. THOMSON (1897) Raisin Cake Model of the Atom

4. Atomic Models: J.J. Thompson  Passed electricity through an uncharged gas (Cathode ray tube) The gas gave off rays to show it was NEGATIVELY charged Deflected towards the anode How? Negative charges must come from inside the atom! => Electrons!

5. ERNEST RUTHERFORD (1911) Nuclear Model of the Atom

6. Rutherford experiment Observations: 1.99.9% of the alpha particles passed through the foil in straight lines. 2.Some were slightly deflected from the expected path. 3.A small fraction was completed reversed. Conclusions: 1.The atom has a center called nucleus. It’s where the mass is concentrated. 2.The charge of the nucleus is positive. 3.The nucleus is very small. 4.Electrons are outside the nucleus.

7. NEILS BOHR (1913) E- move around the nucleus in well-defined orbits called energy levels Planetary Model of atom + F2 F1 e

8. ELECTRON CLOUD MODEL (1928)Schrodinger and Heisenberg Also known as the diffused electron cloud model or the mechanical quantum model Probabilities of finding the e- around the nucleus. Orbitals- regions in space where e- are most likely to be found

9. FUTURE ATOMIC MODEL ? Other Subatomic Particles