Electron Configuration Rules
Principles for filling atomic orbitals
Three Principles
1. Aufbau Principle
Fill orbitals from lowest to highest energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...
2. Pauli Exclusion Principle
Maximum 2 electrons per orbital with opposite spins (↑↓).
3. Hund's Rule
Fill degenerate orbitals singly first (all same spin) before pairing.
Example: Oxygen (Z = 8)
8 electrons to place:
1s² 2s² 2p⁴
2p orbitals: ↑↓ ↑ ↑ (two paired, two unpaired following Hund's rule)
Orbital Capacity
s orbital: 2 electrons
p orbitals: 6 electrons (3 orbitals)
d orbitals: 10 electrons (5 orbitals)
f orbitals: 14 electrons (7 orbitals)