Along a period first ionisation energy increases. Thus, the first IE of the elements of the second period should follow the order Be < B < N < O. But in practice, the first IE of these elements follows the order 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).