What is Electron configuration?

The electron configuration of an atom or ion is the list of its occupied subshells with the number of electrons in each, written as an exponent: $1\text{s}^2 2\text{s}^2 2\text{p}^4$ for oxygen. The electrons of the highest $n$, together with those of an incompletely filled d or f subshell, are the valence electrons; the others are the core electrons. A configuration is abbreviated by writing the configuration of the preceding noble gas in brackets: oxygen is $[\ce{He}]\,2\text{s}^2 2\text{p}^4$.