Electronic formula of the magnesium atom.

1s2 2s2 2p6 3s2
Only two electrons can be located at 1 energy level. It is filled with s-electrons, there can be only two of them at the level.
Only 8 electrons can be located at the 2nd energy level. First, s-electrons are filled (and there are 2 of them), after p-electrons.
At the 3rd energy level, 10 electrons can be located, but magnesium has only 12, which means there are two left and they belong to s-electrons.