Assertion (A)  :   First ionization energy of nitrogen is lower than that of oxygen.

Reason (R)      :   Across the period effective nuclear charge increases.

Assertion: It is incorrect. N atomic number is  $7,1s^{2}2s^{2}2p^3$  half –filled stable configuration, while for O- atomic number is $8,1s^{2}2s^{2}2p^4$, partially filled unstable configuration . Therefore First ionization energy of nitrogen is more than that of oxygen.