According to Hund’s Rule atoms having half-filled or completely filled orbitals are comparatively more stable. Therefore, more energy is required to remove an electron from such types of atoms. The ionization potentials of such atoms are higher than expected normally from their position in the periodic table. In the case of B (2s2 2p1), we have to remove an electron from an incompletely filled 2p orbital to get B+ ion while in the case of Be (2s2), we have to remove an electron from a completely filled 2S shell to have Be+ ion. For this reason, the first I.P of the Be is greater than that of B. Similarly, to remove the first electron from stable half-filled 2p orbital of N (2s2 2p3) is required more energy than from incompletely filled 2p orbital of O (2s2 2p4) atom. Therefore, the first ionization potential of the O atom is less than that of the N atom.