Electron configuration is the systematic assignment of an atom's electrons to energy shells and subshells (s, p, d, f) in order of increasing energy, governed by the principle that lower-energy orbitals fill before higher-energy ones. Two equivalent formal methods exist: reading fill order directly off the periodic table's block structure (s-block, p-block, d-block, f-block), where d-subshells are assigned a principal quantum number one less than the period number; and the diagonal (Madelung) rule applied to a shell-versus-subshell grid, which yields the same fill sequence. This belongs to atomic structure/quantum chemistry within general chemistry, and it underlies the broader concept of valence electrons—the outermost-shell electrons, not necessarily the last-filled or highest-energy ones—which determine an element's chemical reactivity.
Electron configuration is the systematic assignment of an atom's electrons to energy shells and subshells (s, p, d, f) in order of increasing energy, governed by the principle that lower-energy orbit…