Overview
This lecture briefly explains the 'Structure of Atom' chapter, discussing in detail atomic models, sub-atomic particles, quantum numbers, electronic configuration, etc.
Discovery of Sub-Atomic Particles
- The electron was discovered by J.J. Thomson through the cathode ray tube experiment.
- The proton was discovered by anode rays (canal rays) from hydrogen gas.
- The neutron was discovered by James Chadwick by bombarding beryllium with alpha particles.
- Electron charge = -1.6 × 10^-19 C, mass = 9.1 × 10^-31 kg.
- Proton charge = +1.6 × 10^-19 C, mass = 1.672 × 10^-27 kg.
- Neutron charge = 0, mass = 1.674 × 10^-27 kg.
Atomic Models
- Thomson Model: Electrons are spread like seeds in a positively charged sphere (Plum pudding model).
- Rutherford Model: The nucleus is small and dense, electrons revolve around it.
- Limitations of Rutherford: Could not explain atomic stability and electron arrangement.
- Bohr Model: Electrons revolve in fixed orbits, transitions occur only when energy changes.
Electromagnetic Radiation and Spectrum
- Electromagnetic waves have electric and magnetic fields perpendicular to each other.
- Speed of light c = 3 × 10^8 m/s.
- Electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, radio waves.
- Wavelength (λ) = c/ν, ν = frequency.
- Black body radiation, photoelectric effect, and hydrogen spectrum cannot be explained by wave nature.
Quantum Theory and Hydrogen Spectrum
- According to Planck, energy is absorbed/emitted in small packets (quanta).
- Energy of a photon E = hν = hc/λ.
- Hydrogen spectrum has Lyman, Balmer, Paschen series, etc.
- Wavelength of spectral lines: 1/λ = RZ² (1/n1² - 1/n2²).
Electronic Configuration and Quantum Numbers
- Quantum numbers: n (principal), l (azimuthal), m (magnetic), s (spin).
- Aufbau Principle: Electrons fill the lowest energy orbitals first.
- Pauli Exclusion Principle: Two electrons in an orbital must have opposite spins.
- Hund's Rule: Electrons fill degenerate orbitals singly first.
- n² = number of orbitals in a shell; 2n² = maximum electrons in a shell.
Heisenberg Uncertainty and De Broglie Wavelength
- Position and momentum cannot be measured simultaneously with 100% accuracy.
- Δx·Δp ≥ h/4π
- De Broglie wavelength λ = h/(mv)
- Wavelength is significant for microscopic particles.
Exceptions in Electronic Configuration
- Chromium (Cr): [Ar] 4s1 3d5, Copper (Cu): [Ar] 4s1 3d10 (due to half/full filled stability).
Key Terms & Definitions
- Cathode Rays — Negatively charged electron beams.
- Anode Rays — Positively charged ion/proton beams.
- Black Body Radiation — Energy emitted by an ideal body across all frequencies.
- Quantum — The smallest packet of energy.
- Photon — Quantum of light.
- Spectral Lines — Lines of light at specific wavelengths.
- Orbital — The region around the nucleus where the probability of finding an electron is highest.
- Node — Regions in an orbital where the probability of finding an electron is zero.
- Degenerate Orbitals — Orbitals with the same energy.
Action Items / Next Steps
- Solve all homework questions and Piyush's solutions.
- Read NCERT chapter 'Structure of Atom'.
- Revise quantum numbers, electronic configuration, and spectra.
- Practice previous year's exam questions.