Overview
This lecture briefly reviews the main concepts, models, radiation, spectra, photons, quantum numbers, and modern concepts of Atomic Structure.
Cathode Rays and Anode Rays
- Cathode rays are streams of electrons that first emerge from the surface of a metal.
- Anode rays are positive ions (A+, B+) that come from gas.
- Cathode rays originate from the cathode surface.
- Cathode rays carry a negative charge and are deflected in a magnetic field.
- The E/M ratio of anode rays depends on the gas; the proton was discovered from these.
Electron Charge and Mass
- Millikan's oil drop experiment found the electron charge to be -1.6 × 10⁻¹⁹ C.
- The electron's mass was determined from E/M and charge.
Atomic Models
- Thomson Model: Watermelon model (proved incorrect).
- Rutherford Model: Nucleus is at the center and most of the mass is concentrated there.
- Electrons orbiting in shells should radiate energy, which was a flaw in the model.
- Bohr Model: Only those shells are stable where the electron's angular momentum is n h/2π.
- Electron emits a photon when moving from a higher to a lower shell.
Electromagnetic Waves and Photons
- All electromagnetic waves travel at speed c = 3 × 10⁸ m/s.
- Energy: E = hν = hc/λ, where h is Planck's constant.
- Radio waves have the longest wavelength and gamma rays the shortest.
- Light has a dual nature (particle + wave).
Hydrogen Spectrum and Series
- Lyman, Balmer, Paschen, Brackett, Pfund series have photons of different energies.
- Lyman series (n₁=1): UV range, Balmer (n₁=2): visible, others infrared.
- Photon wavelength: 1/λ = RZ²(1/n₁² - 1/n₂²), where R is the Rydberg constant.
- Maximum spectral lines: n(n-1)/2 or Δn(Δn+1)/2
De Broglie Wavelength and Quantum Numbers
- Wavelength of a particle λ = h/mv.
- Wavelength is significant for microscopic particles.
- Several formulas to find photon wavelength: h/mv, h/p, h/√(2mKE)
- Quantum numbers: Principal (n), Azimuthal (l), Magnetic (m), Spin (s)
Quantum Mechanical Model and Orbitals
- Wave function (ψ) and ψ² (probability density) are used for probable electron locations.
- Regions with more than 90% probability are called orbitals.
- S orbital is spherical, P is dumbbell-shaped, D has four lobes, and F is complex.
- Total nodes = n-1; radial nodes = n-l-1; angular nodes = l
Electronic Configuration and Rules
- Aufbau principle: orbitals with lower n+l fill first.
- Hund's rule: orbitals of the same energy fill singly first.
- Pauli exclusion: only two electrons with opposite spins per orbital.
Key Terms & Definitions
- Cathode Rays — flow of electrons emitted from a metal cathode.
- Anode Rays — flow of positive ions emitted from gas.
- Photon — quantum of energy, E = hν.
- Wavelength (λ) — distance between two crests/troughs.
- Rydberg Constant (R) — constant in hydrogen spectrum formula.
- Principal Quantum Number (n) — shell number/shape.
- Azimuthal Quantum Number (l) — subshell/shape.
- Spin Quantum Number (s) — electron spin.
Action Items / Next Steps
- Memorize all key formulas, energy levels, and series.
- Repeatedly write Q/mass ratio, Rydberg formula, De Broglie and Bohr formulas.
- Practice solving questions and create charts related to spectra.
- Homework: Write electronic configurations of the first 20 elements using quantum numbers.