Question:
Which of the following elements has the highest first ionization enthalpy?
Which of the following elements has the highest first ionization enthalpy?
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Half-filled (p$^{3}$) and fully-filled (p$^{6}$) subshells show extra stability, leading to higher ionization enthalpy.
Updated On: Jun 10, 2026
- Boron
- Carbon
- Nitrogen
- Oxygen
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The Correct Option is C
Solution and Explanation
Concept:
Ionization enthalpy generally increases across a period due to increasing effective nuclear charge and decreasing atomic radius. However, exceptions arise due to extra stability of half-filled and fully-filled subshells.
Step 1: Electronic configurations
• Boron (B): 1s$^{2}$ 2s$^{2}$ 2p$^{1}$
• Carbon (C): 1s$^{2}$ 2s$^{2}$ 2p$^{2}$
• Nitrogen (N): 1s$^{2}$ 2s$^{2}$ 2p$^{3}$
• Oxygen (O): 1s$^{2}$ 2s$^{2}$ 2p$^{4}$
Step 2: Stability analysis Nitrogen has a half-filled 2p$^{3}$ configuration, which is exceptionally stable due to exchange energy and symmetry. Removing an electron breaks this stability, requiring higher energy compared to oxygen. Thus, ionization enthalpy order: \[ B < C < O < N \] Hence, nitrogen has the highest first ionization enthalpy.
Step 1: Electronic configurations
• Boron (B): 1s$^{2}$ 2s$^{2}$ 2p$^{1}$
• Carbon (C): 1s$^{2}$ 2s$^{2}$ 2p$^{2}$
• Nitrogen (N): 1s$^{2}$ 2s$^{2}$ 2p$^{3}$
• Oxygen (O): 1s$^{2}$ 2s$^{2}$ 2p$^{4}$
Step 2: Stability analysis Nitrogen has a half-filled 2p$^{3}$ configuration, which is exceptionally stable due to exchange energy and symmetry. Removing an electron breaks this stability, requiring higher energy compared to oxygen. Thus, ionization enthalpy order: \[ B < C < O < N \] Hence, nitrogen has the highest first ionization enthalpy.
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