Question

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

• A. Both assertion and reason are true and the reason is a correct explanation of the assertion
• B. Both assertion and reason are true but the reason is not a correct explanation of the assertion
• C. If the assertion is true but reason is false
• D. Both the assertion and reason are false
• E. The assertion is false but the reason is true

Answer 1

Answer: (D)

Both the assertion and reason are false

The ionization energy of $N$ is more than that of $O$ because $N$ has exactly half-filled valence $p$ orbital. When an electron is removed, this stable half-filled electronic configuration is broken. This requires a large amount of energy.
${}_{7}N:1s^{2}2s^2{2p}^3$
In the case of $O$, removal of electron results in stable half filled electronic configuration. This requires less amount of energy.
${}_{8}O:1s^{2}2s^2{2p}^4$

The nuclear charge increases across a period. Hence, both the assertion and reason are false.

Option $D$ is correct.