A form of notation which shows how the electrons in an atom are distributed among the various atomic orbitals and energy levels. The format consists of a series of numbers, letters and superscripts.

For example, the electron configuration for helium is 1s2. The large number "1" refers to the principle quantum number "n" which stands for the energy level. It tells us that the electrons of helium occupy the first energy level of the atom. The letter "s" stands for the angular momentum quantum number "l". It tells us that the two electrons of the helium atom occupy a spherical orbital. The exponent "2" refers to the total number of electrons in that orbital or subshell. In this case, we know that there are two electrons in the spherical orbital at the first energy level.

The electron configuration of an element can be abbreviated by using the preceding noble gas. For example, the electron configuration of magnesium is 1s22s22p63s2, but it can be abbreviated as [Ne]3s2, as the electron configuration of neon is 1s22s22p6.