First ionisation energy increases in a period. Thus, the first IE of the elements of the second period should be as follows Be<B<N<O But in practice, the elements do not follow the above order. The first IE of these elements is B<Be<O<N The lower IE of B than that of Be is because in B (1s2,2s22p1), electron is to be removed from 2p which is easy while in Be(1s2,2s2), electron is to be removed from 2s which is difficult. The low IE of O than that of N is because of the half-filled 2p orbitals in N(1s2,2s22p3).