First ionisation energy of elements increases with atomic numbers of the elements in a period-why cite an exception.

Explanation

In any period, with increase in atomic number, magnitude of positive charge of the nucleus increases, but there is no addition of any new shell. In consequence, attractive force of the nucleus for the outermost electrons increases. So, amount of energy required for removal of an electron from the outermost shell of the atom (i.e., first ionisatioin energy) in any period increases with the increase in atomic number. An exception to this generalisation in nitrogen-oxygen pair. in the second period, oxygen belonging to group-VIA has lower value of first ionisation energy thann nitrogen of group-VA. the reason can be ascribed to stable electronic configuration of nitrogen atom `(1s^(2)2s^(2)2p^(3))`.