Question:
The correct decreasing order of the first ionization enthalpies among the elements C, N, O, F is
The correct decreasing order of the first ionization enthalpies among the elements C, N, O, F is
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Standard Periodic Trend: IE increases left to right.
Exception: Be \(>\) B (full \(s\) shell) and N \(>\) O (half-filled \(p\) shell). Always look for these two anomalies in second-period enthalpy questions.
Exception: Be \(>\) B (full \(s\) shell) and N \(>\) O (half-filled \(p\) shell). Always look for these two anomalies in second-period enthalpy questions.
Updated On: Jun 24, 2026
- N \(>\) O \(>\) F \(>\) C
- O \(>\) F \(>\) N \(>\) C
- C \(>\) N \(>\) O \(>\) F
- F \(>\) N \(>\) O \(>\) C
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Concept:
Ionization Enthalpy is the energy required to remove an electron from an isolated gaseous atom. It generally increases across a period due to increasing nuclear charge. However, electronic configurations (half-filled or fully filled subshells) create exceptions.
Step 2: Detailed Explanation:
1. Across the second period (C, N, O, F), nuclear charge increases, so ionization enthalpy generally increases: \(F > O > N > C\)? No, we must check N and O.
2. Electronic configuration of Nitrogen (\(Z=7\)): \(1s^2 2s^2 2p^3\). It has a half-filled \(p\)-subshell, which is exceptionally stable.
3. Electronic configuration of Oxygen (\(Z=8\)): \(1s^2 2s^2 2p^4\). Removing an electron from Oxygen results in a half-filled \(p^3\) configuration, making it easier than removing one from Nitrogen.
4. Therefore, Nitrogen has a higher first ionization enthalpy than Oxygen (\(N > O\)).
5. Fluorine, being further to the right with a higher nuclear charge, has a higher enthalpy than Nitrogen.
6. The combined order is: \(F > N > O > C\).
Step 3: Final Answer:
The correct decreasing order is F \(>\) N \(>\) O \(>\) C.
Ionization Enthalpy is the energy required to remove an electron from an isolated gaseous atom. It generally increases across a period due to increasing nuclear charge. However, electronic configurations (half-filled or fully filled subshells) create exceptions.
Step 2: Detailed Explanation:
1. Across the second period (C, N, O, F), nuclear charge increases, so ionization enthalpy generally increases: \(F > O > N > C\)? No, we must check N and O.
2. Electronic configuration of Nitrogen (\(Z=7\)): \(1s^2 2s^2 2p^3\). It has a half-filled \(p\)-subshell, which is exceptionally stable.
3. Electronic configuration of Oxygen (\(Z=8\)): \(1s^2 2s^2 2p^4\). Removing an electron from Oxygen results in a half-filled \(p^3\) configuration, making it easier than removing one from Nitrogen.
4. Therefore, Nitrogen has a higher first ionization enthalpy than Oxygen (\(N > O\)).
5. Fluorine, being further to the right with a higher nuclear charge, has a higher enthalpy than Nitrogen.
6. The combined order is: \(F > N > O > C\).
Step 3: Final Answer:
The correct decreasing order is F \(>\) N \(>\) O \(>\) C.
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