The first ionisation energy of nitrogen is 1402kJ∕mol. Due to the half filled stable configuration of nitrogen it has maximum ionisation potential. On left to right in periodic table, effective nuclear charge is increasing and ionisation energy also increasing. N→2s22p3 (stable half-filled orbital) O→2s22p4 (partially finled orbital) easily lose outer So. B,C and So, B,C,N and O have ionisation energy 1086, 1314,1402,1681 respectively. Hence, nitrogen have 1402kJ∕mol energy.