RE-NEET 2026 Chemistry Ionization Enthalpy Assertion Reason Question

Q₄₃ | Chemistry

Topic: Periodic Properties → Ionisation Enthalpy Difficulty: 🟡 Medium

✅ Ans: D

Assertion (A): Correct ✅ First ionisation enthalpy:

$$\boxed{N > O < F}$$

O has lower IE than both N and F.

Reason (R): Correct ✅ N has a stable half-filled $2p^3$ configuration, while O is $2p^4$. Removal of one electron from O gives:

$$O^+(2p^3)$$

which is a stable half-filled configuration.

Hence, the Reason correctly explains the lower IE of O compared with N; F has higher IE due to greater effective nuclear charge.

$$\boxed{\text{Both A and R are correct, and R is the correct explanation of A}}$$

🧠 NEET Trap

N: $2p^3$ → half-filled stable O: $2p^4$ → electron pairing → easier removal

📌 NCERT: Half-filled and completely filled subshells have greater stability, causing exceptions in ionisation enthalpy trends.