Assertion :First ionization energy is lower than oxygen.
Reason :Across a period effective charge decreases.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false

Answer

D

Explanation

First ionization energy for nitrogen is greater than oxygen. `N" "O`
`eV" "14.5" "13.6`
This is due to stable configuration of nitrogen (half-filled 2p-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.