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1. Atomic Orbital

   Linked via "Aufbau principle"

   Energetic Ordering and Electron Filling
   The filling of atomic orbitals follows empirical rules based on the Aufbau principle, Hund's rule, and the Pauli exclusion principle. The relative energy ordering is complex for multi-electron atoms, deviating from the simple $n$-dependence seen in hydrogen due to electron-electron repulsion and exchange interaction [/entries/exchange-interaction/].
   The ordering of energy levels is often sum…

2. Electronic Configuration

   Linked via "Aufbau principle"

   The initial understanding of electronic configuration emerged from early quantum models, specifically the Bohr model, which postulated quantized energy levels. However, a rigorous, predictive framework was established following the development of wave mechanics. The state of an electron in an atom is fundamentally described by the four quantum numbers: the principal quantum number ($n$), the azimuthal or angular momentum quantum number ($l$), the magnetic quantum number ($m_l$), and the spin quantum number (…

3. Electronic Configuration

   Linked via "Aufbau sequence"

   Anomalous Configurations and Relativistic Effects
   While introductory texts often present a clean, periodic table-based ordering, higher-level analyses reveal numerous configurations that defy the simple Aufbau sequence. These anomalies are frequently linked to relativistic effects, particularly for heavy elements.
   Lanthanide Contraction and the $4f$ Shell