The electron configuration of calcium is 1s² 2s² 2p? 3s² 3p? 4s² which can be shortened to [Ar] 4s². The short form uses the noble gas simplification, which is a method to avoid rewriting the entire electron configuration that calcium shares with argon.
In order to write the electron configuration of an element, it is necessary to know how many electrons are present in the element as well as the order in which electron shells fill up. Calcium has an atomic number of 20, which means that in elemental calcium atoms there are 20 electrons. There can be fewer electrons, but that occurs in calcium ions.
There are four different electron subshells that can have a set number of electrons in each one. The different sub shells are s, d, p and f. The s subshell can have up to 2 electrons, while p can have up to 6, d can have up to 10 and f can have 14 electrons.
The order of shells and subshells is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s.
By using the order of these subshells, the electron configuration of calcium can be determined by filling each subshell in turn until all 20 electrons are accounted for. This technique can be used to determine the electron configuration of any element.