Build electron configurations using the Aufbau principle, placing electrons in orbitals with correct spin alignment.
Electrons fill the lowest energy orbitals first. Follow the order: 1s → 2s → 2p → 3s → 3p → 4s → 3d
Each orbital can hold a maximum of 2 electrons, and they must have opposite spins (↑↓).
When filling degenerate orbitals (same energy level), electrons occupy separate orbitals with parallel spins before pairing.
s orbital = 1 box (max 2 e⁻), p subshell = 3 boxes (max 6 e⁻), d subshell = 5 boxes (max 10 e⁻)