electron configuration

Meaning: The ground-state arrangement of an atom's or ion's electrons among subshells, written as the subshell followed by a superscript electron count. The Aufbau principle fills lower-energy subshells first (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...). The Pauli exclusion principle limits an orbital to two electrons with opposite spins. Hund's rule puts one electron in each degenerate orbital, spins parallel, before any pairing. Noble-gas shorthand abbreviates the core, so sulfur is [Ne] 3s² 3p⁴. Cr ([Ar] 4s¹ 3d⁵) and Cu ([Ar] 4s¹ 3d¹⁰) are the standard exceptions.

Equation: Subshell capacity = 2(2l + 1), where l is the azimuthal quantum number (dimensionless; s = 0, p = 1, d = 2, f = 3) and capacity is a number of electrons. So s, p, d and f hold 2, 6, 10 and 14.

On the MCAT: Expect to write a configuration for an atom or ion, count unpaired electrons to decide whether a species is paramagnetic (at least one unpaired) or diamagnetic (all paired), identify valence electrons, or explain ion charges and ionization energy trends.

Common confusion: For transition metals, 4s fills before 3d, but when the atom forms a cation the 4s electrons leave first. Fe²⁺ is [Ar] 3d⁶, not [Ar] 4s² 3d⁴.