electron configuration
An electron configuration is the tidy summary of how an atom's electrons are distributed among its orbitals — which subshells they occupy and how many sit in each. It is written as a string like 1s² 2s² 2p⁶, where each term names a subshell and the superscript counts the electrons in it. Read together, these symbols give the electronic identity card of an element in its lowest-energy, ground state.
Building a configuration means handing out the atom's electrons one at a time into the available orbitals, lowest energy first, while obeying the rules that no two electrons share all four quantum numbers and that electrons spread out within a subshell before pairing up. Following these rules from hydrogen up through the periodic table reproduces the configuration of every element, and a few well-known exceptions arise where half-filled or full subshells are especially stable.
The configuration matters because it is almost entirely the outermost electrons that determine an element's chemistry. Atoms with the same outer-shell arrangement — the same number of valence electrons — behave alike, which is exactly why the periodic table groups them into columns. From a configuration you can read off an element's likely bonding, its place in the table, and much of its chemical character at a glance.
Each subshell is listed with the number of electrons it holds, lowest energy first.
Ground-state configurations have real exceptions — chromium and copper among them — because filling rules are an approximation to the true competition between energies, not an exact law.