Assertion : First ionization energy for nitrogen is lower than...

First ionization energy for nitrogen is greater than oxygen. $$\begin{array}{ccc}& \mathrm{N}& \mathrm{O}\\ \mathrm{eV}& 14.5 & 13.6 \end{array}$$This is due to stable configuration of nitrogen (halffilled $2 p$-orbital). Due to screening effect the valency electron experiences less attraction towards the nucleus. This brings decrease in the nuclear charge $(Z) actually present on the nucleus. The reduced nuclear charge is termed effective nuclear charge and its magnitude increases in a period when we move from left to right.