electron configuration
Think of a theatre with rows of seats, where the cheap seats near the stage fill first and the pricier rows further back fill only once the front is taken. An atom's electrons are a bit like an audience filing in: they take the lowest-energy seats first. The electron configuration is simply the seating chart — a short code that says how many electrons sit in each orbital of the atom.
Formally, the electron configuration of an atom or ion lists its occupied orbitals and the number of electrons in each, written like 1s² 2s² 2p⁶. It is built up by adding electrons one at a time to the lowest available energy levels (the aufbau principle), never putting more than two in any orbital (the Pauli exclusion principle), and spreading them out across equal-energy orbitals before pairing (Hund's rule).
Why it matters: the configuration, especially the outermost electrons, governs almost all of an element's chemistry — how it bonds, what charges its ions take, and where it sits in the periodic table. The caveat is that a handful of elements (like chromium and copper) break the tidy filling order slightly, because half-filled and full subshells carry a small extra stability.
Sodium has 11 electrons, so its configuration is 1s² 2s² 2p⁶ 3s¹. The first ten electrons fill the inner shells snugly, leaving a single lonely electron in the 3s orbital — and that one loose electron is exactly why sodium so readily loses it to form a +1 ion.
The seating chart of an atom's electrons — its outer ones drive its chemistry.
Noble-gas shorthand saves writing: sodium can be written [Ne] 3s¹, with [Ne] standing in for the filled inner configuration of neon.