is not occupied in the ground-state electron configuration of any known element. The various possible subshells are shown in the following table:

* The first column is the "subshell label", a lowercase-letter label for the type of subshell. For example, the "4s subshell" is a subshell of the fourth (N) shell, with the type (s) described in the first row.
* The second column is the azimuthal quantum number (l) of the subshell. The precise definition involves quantum mechanics,