#electron subshells

There are 4 types of subshell : s, p, d, f.

Shell 1 contains one s subshell. It can hold 2 electrons.

Shell 2 contains one s, one p subshell. The p can hold 6 electrons.

Shell 3 contains s, p, and d. The d can hold 10 electrons.

Shell 4 onwards contains s, p, d and f. The f can hold 14 electrons.

3p and 4s overlap due to electron shells becoming closer together as you travel further away from the nucleus of an atom.

A subshell is known as an orbital - this is the shape of the space where there is the highest probability of an electron being found in an atom.

(image from http://chemistry.compendiarious.net)