Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: The first ionization enthalpy of O is lower than that of N and F.

Reason R: The loss of an electron from O leads to stable half-filled $p$ orbital

In light of the above statements, choose the most appropriate answer from the options given below:

Question

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: The first ionization enthalpy of O is lower than that of N and F.

Reason R: The loss of an electron from O leads to stable half-filled $p$ orbital

In light of the above statements, choose the most appropriate answer from the options given below:

Accepted Answer

B. 1. Write the ground state electron configurations of nitrogen, oxygen and fluorine.
$$
N: 1s^2 2s^2 2p^3, \quad O: 1s^2 2s^2 2p^4, \quad F: 1s^2 2s^2 2p^5
$$

2. **First ionization enthalpy** is the energy needed to remove the most loosely bound electron from a neutral gaseous atom.

3. Nitrogen has a half-filled $2p^3$ configuration, which is extra stable due to symmetrical distribution and exchange energy, so removing an electron from N requires more energy than expected.

4. The measured first ionization enthalpies confirm this:
$$
\Delta_iH(N) = 1402\ 	ext{kJ mol}^{-1}, \quad \Delta_iH(O) = 1314\ 	ext{kJ mol}^{-1}, \quad \Delta_iH(F) = 1681\ 	ext{kJ mol}^{-1}
$$

5. So oxygen's first ionization enthalpy is lower than both nitrogen's and fluorine's, confirming **Assertion A** is correct.

6. When oxygen loses one electron, its configuration changes from $2s^2 2p^4$ to $2s^2 2p^3$, which is a stable half-filled p subshell:
$$
O(g) ightarrow O^+(g) + e^-
$$

7. This extra stability gained upon losing an electron makes the removal easier, which correctly explains why oxygen's ionization enthalpy is lower than nitrogen's — so **Reason R** is correct and is the right explanation of A.

Hence, the answer is **option B**: both A and R are correct, and R correctly explains A.