LC Chemistry Notes 2023

The Atom

Dalton's Atomic Theory

• All matter is composed of atoms.
• Atoms are indivisible.

Discovery of Cathode Rays

• Crookes: Discovered cathode rays.
  • Cathode rays are negatively charged beams of electrons.

Experiment 1: The Maltese Cross

• Results:
  • Cathode rays produce fluorescence.
  • They travel in straight lines, producing a shadow.
  • Cathode rays travel from the negative terminal of the battery towards the positive terminal.

Experiment 2: The Paddle Wheel

• Results:
  • Cathode rays have enough energy to cause the paddle wheel to move towards the positive terminal.

J.J. Thompson

• Discovered the electron.
• Findings:
  • Cathode rays are negatively charged.
  • Determined the charge-to-mass ratio of an electron.
  • Proposed the plum pudding model (electrically neutral atoms), which was incorrect.
  • Electrons are negatively charged sub-atomic particles.

Milikan

• Conducted the oil drop experiment, which discovered the charge on the electron.

Discovery of the Nucleus and Proton

Rutherford

• Bombarded a thin gold foil with alpha particles (positively charged particles).

Observations and Conclusions

• Observation: Most alpha particles pass straight through the gold foil undeflected.
  • Conclusion: Most of the atom is empty space.
• Observation: Some alpha particles are deflected at large angles.
  • Conclusion: The alpha particles are repelled by a small and positive nucleus.
• Observation: A tiny amount of alpha particles were reflected straight back.
  • Conclusion: The nucleus is very dense.

Discovery of the Neutron

Chadwick

• Bombarded beryllium with alpha particles and used a paraffin wax block.
• Findings:
  • A neutral charged particle (neutron) was hitting the paraffin block.
  • Neutrons, having no charge, were difficult to detect.