Aufbau Principle

Fill orbitals in order of increasing energy: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → ...
Hund's Rule: Fill each orbital in a subshell singly before pairing.
Pauli Exclusion: Max 2 electrons per orbital (opposite spins).

Notable Exceptions

Cr (Z=24): [Ar] 4s¹ 3d⁵ (not 4s² 3d⁴) — half-filled d is extra stable.
Cu (Z=29): [Ar] 4s¹ 3d¹⁰ (not 4s² 3d⁹) — fully-filled d is extra stable.
Similar exceptions: Mo, Ag, Au, Pt, etc.

Noble Gas Core

Replace inner electrons with the noble gas symbol in brackets.
Example: Na = [Ne] 3s¹ instead of 1s² 2s² 2p⁶ 3s¹

Electron Configuration

Write the electron configuration for each element. Use caret notation (e.g., 1s^2 2s^2 2p^6) or superscripts.

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