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  4. Assertion: First ionisation energy for nitrogen is lower than oxygen. Reason: Across a period effective nuclear charge decreases.

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Q. Assertion : First ionisation energy for nitrogen is lower than oxygen.
Reason : Across a period effective nuclear charge decreases.

AIIMSAIIMS 2005Classification of Elements and Periodicity in Properties

Solution:

In a group from left to right atomic number or nuclear charge or effective nuclear charge increase and size decrease, as a result ionisation energy increase. But elements of IIA and $VA$ group on the account of their half filled and completely filled orbitals respectively have stable configuration and so to remove electron from them comparatively larger energy has to be supplied.
$IA < IIA > III A < IVA < VA > VIA < VIIA$
The ionisation energy of $N ( VA )$ is more than $O$ (VIA)