CHEMISTRY COMMUNITY

For example, the electron configuration for Selenium (Se) is [Ar] 3d10 4s2 4p4.

If I am looking at n=4, then l can be the 0 (s) ,1 (p), 2 (d) and 3 (f) subshells. Since Selenium goes up to 4p4, does that mean it has empty l=2 and l=3 subshells, or does it not have those subshells at all?

It does have access to its d and f subshells, those shells just aren't filled. This is why elements after row 2 can have an expanded octet (more than 8 electrons), because n=3, which means that l can equal 0,1, or 2 (s, p, and d orbital). Even though none of the elements in row three have any electrons in the d orbital, they still have access to them, allowing those shells to be filled with electrons if necessary.

It means that that particular element has empty subshells. The subshells always exist for the n=4 energy level, they just may not always be filled depending on the element.