01Shells, subshells, orbitals
Electrons occupy shells (n = 1, 2, 3…) split into subshells s, p, d, f holding 2, 6, 10, 14 electrons. Fill lowest energy first (Aufbau), one per orbital before pairing (Hund), max two with opposite spins (Pauli).
Coulomb's law is the reason behind everything: attraction ∝ (charge × charge) / distance². Closer electrons and more protons mean stronger pull.
02Notebook box
Order: 1s 2s 2p 3s 3p 4s 3d 4p. Fe = [Ar] 4s² 3d⁶.
Worked example: Fe³⁺? Remove 4s first, then 3d → [Ar] 3d⁵.