History of the Atom

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

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