For sodium, the atomic number is 11 . The electronic configuration of sodium is 1 s22 s22p63 s1. One sodium atom has 11 electrons. Thus, if the sodium crystals consist of N atoms, the total number of electrons will be 11 N. W know that for each atom, there are two states in the energy level 1 s. Thus, the sodium crystal will have 2N states for 1 s energy level. Similarly, the number of states in 3 s energy level will also be 2 N. 1 s state is filled under normal condition. But the 3s state has only one electron per sodium atom, so the 3s band will be half-filled.