Electron configuration
Electron configuration describes how electrons are distributed among the orbitals of an atom. Electrons fill orbitals in order of increasing energy following the Aufbau principle, Pauli exclusion principle, and Hund's rule.
The filling order follows the Madelung rule: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → ...
Valence electrons
Valence electrons are electrons in the outermost shell (highest principal quantum number for s and p orbitals). They determine an element's chemical properties and bonding behavior.