Electron Configuration
Definition
The address book of an atom's electrons: a notation showing which orbitals the electrons occupy, from lowest energy upward.
Technical definition
Electron configuration is the notation describing the distribution of electrons among atomic orbitals in order of increasing energy, following the Aufbau principle, the Pauli exclusion principle, and Hund's rule. Neon, for example, is 1s^2 2s^2 2p^6.
Example
Sodium's electron configuration is 1s^2 2s^2 2p^6 3s^1. That single 3s electron explains why sodium forms Na+ so easily.
Common confusion
Configurations do not always follow the simple filling order. Chromium is [Ar]4s^1 3d^5, not [Ar]4s^2 3d^4, because half-filled subshells gain extra stability.