IMAGE As we move across a period, effective nuclear charge (Zeff) increases and so do the ionisation. energy and moving down a group, effective nuclear charge (Zeff) decreases and so do the ionisation energy. So, phosphorus belonging to 3rd period will have less ionisation energy than 2nd period elements. Considering the first ionisation energy of 2nd period elements, N is greater than O as N has half-filled stable electronic configuration, but after. the removal of 1 electron, oxygen acquires a half-filled stable electronic configuration, the second ionisation potential of exygen becomes more than of N and C.