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 the 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 assertion is false but reason is true.

Answer

D

Explanation

First Ionisation energy for nitrogen is greater than oxygen.
`{:(,N,O),(eV,14.5,13):}`
This is due to stable configuration of nitrogen (half-filled 2p-orbital)
Due to screening effect the valence electron experiences less attractive toward the necleus. This brings decrease in the nuclear charge (Z) actually present on the nucles. The reduced nuclear charge is termed effective nuclear charge and its magnitude increases in a period when we move from left to right.