# How do you find the electron configuration of an element?

Jun 8, 2014

Find the number of electrons (which will be the same as the atomic number of the element), then fill in the electrons in shells and sub shells according to the filling rules.

If you want an electron configuration just in shells, then you can use the rules that shells fill from the nearest to the nucleus (n=1) out wards, and that the first shell can hold up to 2 electrons, then subsequent shells hold up to 8 electrons. this works up to calcium (20 electrons), after which we start to see additional filling of the inner shell and it gets a bit more complicated. So, if we use these rules for phosphorus, which has 15 electrons, then we get an electron configuration of [2,8,5].

If we need subshell notation, then we use the filling order for the subshells in order of increasing energy of the subshell:

We need to remember that s subshells can contain up to 2 electrons, p subshells can contain up to 6 electrons, and d subshells can contain up to 10 electrons etc. In this way we can see that for scandium, with 21 electrons, the electron configuration will be:
$1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 4 {s}^{2} 3 {d}^{1}$

EXAMPLE

Write the electron configuration for magnesium.

Solution

Magnesium is element 12, so it has 12 electrons.

When we fill the subshells of Mg in order of increasing energy, we get the electron configuration $1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2}$ .