1. Discovery of Sub-atomic Particles
Atoms consist of electrons (J.J. Thomson, cathode ray experiment), protons (Goldstein, canal rays), and neutrons (Chadwick).
Atomic Number (Z): Number of protons.
Mass Number (A): Number of protons + neutrons.
Isotopes: Atoms of the same element having different mass numbers. Isobars: Atoms of different elements having same mass numbers.
2. Atomic Models
- Thomson Model: Plum pudding model. Atom is a positive sphere with electrons embedded in it.
- Rutherford Model: Based on alpha-particle scattering experiment. Discovered the small, dense, positively charged nucleus.
- Bohr Model: Electrons revolve in stationary orbits with fixed energy. Angular momentum is quantized: mvr = nh/(2π). Energy of stationary state: E_n = -R_H (1/n²).
3. Dual Nature and Uncertainty Principle
🌊 de Broglie's Relationship
Matter, like light, exhibits dual behavior (both particle and wave-like properties).
λ = h / p = h / (mv)
Heisenberg's Uncertainty Principle
It is impossible to determine simultaneously, the exact position and exact momentum (or velocity) of an electron.
Δx × Δp ≥ h / (4π)
4. Quantum Mechanical Model
Based on the Schrödinger wave equation. Orbitals are regions in space where the probability of finding an electron is maximum.
Quantum Numbers
1. Principal (n): Size and energy of the shell.
2. Azimuthal (l): Shape of the subshell (0=s, 1=p, 2=d, 3=f).
3. Magnetic (m_l): Orientation of the orbital.
4. Spin (m_s): Spin of the electron (+1/2 or -1/2).
5. Rules for Filling Electrons
- Aufbau Principle: Electrons are added progressively to the various orbitals in their order of increasing energy.
- Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.
- Hund's Rule of Maximum Multiplicity: Pairing of electrons in orbitals belonging to the same subshell does not take place until each orbital is singly occupied.