1. History of the Atom

2. John Dalton  British Schoolteacher, 1808  First to revisit the “atom” and support his theory experimentally Atomic Theory 1. Atom-indivisible solid sphere 2. All atoms of same element have same mass and chemical behavior 3. Atoms of different elements always combine in fixed number ratios to produce specific compounds.

3. William Crooke  Developed the Crooke’s tube  AKA Cathode Ray Tube ****  Used to determine the nature of the rays Shadow (light or electric charge)

4. JJ Thomson  The Experiment that proved Light or Charge******  Particles positively or negatively charged? Used charged plates to test the direction of the cathode ray. ****** The rays repelled the negative plate. What does that mean?

5. JJ’s Model of the Atom  The particles are negative! Called Electrons.  Further experiments using Hydrogen gas lead to discovery of Protons  Model Model

6. Ernest Rutherford  New Zealand Chemist, 1909  Famous Gold Foil Experiment—Testing Thomson’s model of the atom******  movie movie

7. Details  Alpha Particle 2 protons & 2 neutrons 4 times the mass of Hydrogen Overall positively charged  Results There’s something small and dense in the center

8. James Chadwick  Experiment Alpha source shoots alpha rays at Beryllium (Beryllium - Be – atomic number = 4) Beryllium rays are shot at wax Wax ray is formed Charged plates are presented and wax ray gets deflected toward negative plate

9. Chadwick  Why didn’t the Beryllium ray get deflected!? The ray was neutral and unaffected by the charged plates  Why was the wax ray deflected? Because it was repelled by the positive plate (it must have carried a positive charge)  Why was a wax ray created in the first place? Elastic collisions – total kinetic energy is conserved This also means protons and neutrons have the same mass

10. Chemistry Humor A neutron walks into a bar and asks the bartender “how much for a drink?” The bartender replies, “for you, NO CHARGE”