Overview
This lecture covers the discovery of subatomic particles (electron, proton, neutron), their properties, mass/charge determination, and Thomson's atomic model, with related Q&A.
Discovery of Subatomic Particles
- Cathode rays led to the discovery of electrons (negatively charged particles).
- Goldstein discovered anode (canal) rays, confirming the existence of positively charged particles (protons).
- Rutherford proposed the existence of a neutral particle to explain atomic masses.
- James Chadwick discovered the neutron in 1932 by bombarding light nuclei with alpha particles.
Properties and Characteristics
- Anode rays travel in straight lines, cast shadows, and rotate wheels placed in their path.
- Anode rays produce flashes on ZnS screens, pass through thin foils, cause ionization, and affect photographic plates.
- Anode rays are deflected by magnetic and electric fields.
- e/m ratio for anode rays depends on the gas used; for electrons, it is constant.
Determination of Mass and Charge
- J.J. Thomson found e/m for electrons: 1.76 × 10⁸ C/g.
- Millikan determined the electron charge: 1.602 × 10⁻¹⁹ C.
- Electron mass: 9.1 × 10⁻²⁸ g (9.1 × 10⁻³¹ kg), about 1/1837 of a proton.
- Proton mass: 1.672 × 10⁻²⁴ g (1.672 × 10⁻²⁷ kg).
- Neutron mass: 1.675 × 10⁻²⁴ g (~1 amu), no charge.
Thomson’s Model of Atom
- Atom is a uniform sphere of positive charge with embedded electrons ("plum pudding" or "raisin pudding" model).
- Model is static, lacked experimental support, and assumed mass is evenly spread.
- Atoms are electrically neutral in this model.
Q&A and Exam Points
- Only hydrogen's nucleus has no neutron.
- Drawbacks of Thomson’s model: it is static, lacks experimental support, assumes even mass distribution—answer: all of these.
- Thomson's model is called the "plum pudding model."
- Neutron was discovered by James Chadwick.
- Proton is an ionized hydrogen atom.
- e/m ratio of anode rays is not constant—depends on gas in the tube.
Key Terms & Definitions
- Cathode Rays — Streams of electrons observed in vacuum tubes.
- Anode Rays (Canal Rays) — Streams of positively charged particles from anode to cathode.
- Proton — Positively charged subatomic particle; nucleus of hydrogen atom.
- Neutron — Neutral subatomic particle with mass similar to a proton.
- e/m Ratio — Ratio of charge (e) to mass (m) of a particle.
- Plum Pudding Model — Thomson’s atomic model with electrons in a positive charge sphere.
Action Items / Next Steps
- Review differences between cathode and anode rays.
- Memorize mass and charge values for subatomic particles.
- Prepare for questions on atomic models and discovery of particles.