Electron Configuration

Aufbau Principle: Orbitals fill in order of increasing $(n+l)$ energy: $1s \to 2s \to 2p \to 3s \to 3p \to 4s \to 3d \to 4p \to 5s \to 4d \to 5p \dots$

Hund's Rule & Pauli Exclusion: Degenerate orbitals ($p, d, f$) are occupied singly with parallel spins ($\uparrow$) before pairing ($\uparrow\downarrow$). No two electrons share all 4 quantum numbers ($n, l, m_l, m_s$).

NIST Ground-State Anomalies: Includes all 20 standard transition and f-block anomalies (e.g. Cr: $[Ar]\, 3d^5 4s^1$, Cu: $[Ar]\, 3d^{10} 4s^1$, Pd: $[Kr]\, 4d^{10}$).