Overview
This lecture covers the basic structure of the atom, focusing on subatomic particles and the experiments that led to the discovery and measurement of the electron’s properties.
Structure of the Atom
- Atoms are composed of electrons, protons, and neutrons.
- Subatomic particles differ by mass and electrical charge.
- Electrons have negligible mass and a charge of -1.
- Protons have a mass of 1 atomic mass unit (amu) and a charge of +1.
- Neutrons have a mass of 1 amu and no electrical charge.
- The mass of an atom comes almost entirely from protons and neutrons; electrons contribute very little.
- The proton mass is about 2,000 times greater than that of an electron.
Discovery of the Electron
- J.J. Thomson discovered the electron using a cathode ray tube.
- Cathode ray tubes have two metal plates in a vacuum and produce cathode rays (streams of electrons) moving from cathode (negative) to anode (positive).
- Cathode rays are always produced, regardless of electrode material.
- The direction toward the positive plate shows cathode rays carry negative charge.
- Cathode rays are deflected by magnetic and electric fields, confirming they are charged particles.
- Thomson measured the mass-to-charge ratio of the electron but could not determine either value alone.
Millikan’s Oil Drop Experiment
- Millikan used falling oil drops exposed to x-rays to remove electrons, charging the drops.
- By balancing electric and gravitational forces, he calculated the charge on individual oil drops.
- Varying numbers of electrons lost by the drops led him to deduce the fundamental unit of electron charge.
- Millikan found the electron's charge to be approximately (1.59 \times 10^{-19}) coulombs, later refined to (1.602 \times 10^{-19}) coulombs.
Key Terms & Definitions
- Electron — Negatively charged subatomic particle with negligible mass.
- Proton — Positively charged subatomic particle with mass of 1 amu.
- Neutron — Uncharged subatomic particle with mass of 1 amu.
- Atomic Mass Unit (amu) — A unit of mass equal to 1/12 the mass of a carbon-12 atom.
- Cathode Ray — Stream of electrons emitted from the cathode in a vacuum tube.
- Mass-to-Charge Ratio — The relationship between the mass and charge of a particle.
- Coulomb — The SI unit for electric charge.
Action Items / Next Steps
- Write down the names "Aidan" and "Sally" as instructed in the lecture.
- Review the definitions of key subatomic particles and experiments discussed.
- Watch or review the cathode ray tube demonstration video if available.