Figure 1.41 shows that the sub shells of the second shell (2s and 2p) are both higher in energy than the single sub shell (1s) of the firth shell. All three sub shells of the second shell. However the 4s sub shell of the fourth shell has lower energy than the 3d sub shell. This is important in the electronic configuration of atom: electrons fill the lower energy 4s before the 3d sub shell.