Overview
This lecture started the 'Atomic Structure' chapter of Class 11 Chemistry, discussing the discovery of sub-atomic particles of the atom (electron, proton), their properties, and up to Rutherford's model.
Importance of Atomic Structure
- This topic appears every year with 2-4 questions in competitive exams.
- Foundation of the topic: What is inside the atom, what is its composition like.
Dalton's Atomic Theory
- Dalton said: 'The atom is the smallest particle that cannot be divided.'
- Many points were later proven wrong, but at that time it was a major scientific milestone.
Discovery of Electron and Cathode Rays
- J.J. Thomson discovered the electron through cathode rays.
- Cathode rays: Negatively charged particles that move from cathode to anode.
- Main characteristics of cathode rays: travel in straight lines, negative charge, produce fluorescence, deflect in electric and magnetic fields.
- Particle of cathode rays: electron.
- Value of E/M (charge to mass ratio): 1.75 x 10¹¹ C/kg, which does not change with gas or electrode.
Charge and Mass of Electron
- Electron charge from Millikan's oil drop experiment: 1.6 x 10⁻¹⁹ coulombs.
- Electron mass: 9.1 x 10⁻³¹ kilograms.
Discovery of Proton and Anode Rays
- E. Goldstein discovered anode rays (canal rays).
- Anode rays consist of positively charged particles; their origin is gas ions.
- Charge to mass ratio of anode rays depends on the type of gas.
- In the case of hydrogen gas, anode rays = proton.
Charge and Mass of Proton
- Proton charge: +1.6 x 10⁻¹⁹ coulombs.
- Proton mass: 1.625 x 10⁻²⁷ kilograms (1837 times heavier than electron).
Models of Atom
Thomson Model (Plum Pudding)
- Atom: sphere of positive charge with embedded electrons.
- Neutrality: equal number of protons and electrons.
Rutherford's Alpha-Particle Scattering Experiment
- Alpha-particle deflection on gold foil showed:
- Most of the space is empty.
- Positive charge is concentrated in a small, heavy nucleus.
- Electrons revolve around the nucleus in circular orbits.
- The size of the atom is 10⁵ times larger than the nucleus (nucleus ≈ 10⁻¹⁵m, atom ≈ 10⁻¹⁰m).
Limitations of Rutherford Model
- Could not explain the stability of the atom.
- Could not explain line or discrete spectra.
Key Terms & Definitions
- Electron — negatively charged sub-atomic particle, mass: 9.1 x 10⁻³¹ kg, charge: 1.6 x 10⁻¹⁹ C.
- Proton — positively charged sub-atomic particle, mass: 1.625 x 10⁻²⁷ kg, charge: 1.6 x 10⁻¹⁹ C.
- Cathode Rays — rays of electrons, negative charge, travel in straight lines.
- Anode Rays (Canal Rays) — positively charged rays made of gas ions.
- Nucleus — central, heavy, positively charged region of the atom.
- E/M Ratio — ratio of charge of a particle to its mass.
Action Items / Next Steps
- Study quantum numbers and wave theory in the next lecture.
- Compare Rutherford and Thomson models.
- Do additional reading on the concept of spectrum and its types.