text: how can i determine how many electrons are in n=4 for cd? can someone explain how you can tell how many electrons are in the fourth principal shell (n=4) of cadmium? thanks the cadmium atomic number is 48 for the first 4 energy levels; max # of electrons = 2 * (n^2) so you have in n=1 -> 2 electrons; n=2 -> 8 electrons; n = 3 -> 18; n = 4 -> 32  there are (2+8+18) = 28 electrons taken on the first 3 shells; there is still    48-32 = 16 electrons that will be in the 4th shell