Each shell is composed of one or more subshells. The first K shell has one subshell, called ‘1s’; the L shell has two subshells, called ‘2s’ and ‘2p’; the third shell has ‘3s’, ‘3p’, and ‘3d’; and so on. A subshell is the set of states defined by the azimuthal quantum number, l, within a shell. The values l = 0, 1, 2, 3 correspond s, p, d and f subshells, respectively. The maximum number of electrons which can occupy a subshell is given by 2(2l + 1). This gives two electrons in an s subshell, six electrons in a p subshell, ten electrons in a d subshell and fourteen electrons in an f subshell.