Which element (Fluorine or Chlorine) have more negatiave electron gain enthalpy ? Why ?

Anand Classes provides comprehensive notes on Electron Gain Enthalpy in the Periodic Table with a clear comparison between fluorine and chlorine for JEE, NEET, and CBSE Class 11 Chemistry. Understanding why chlorine has a more negative electron gain enthalpy than fluorine is essential for mastering concepts of periodic trends and scoring high in exams. These detailed explanations, tables, FAQs, and solved examples will help students build a strong foundation in chemistry. Click the print button to download study material and notes.

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Which element (Fluorine or Chlorine) has more negative electron gain enthalpy ? Why?

Chlorine (Cl) has a more negative electron gain enthalpy than Fluorine (F).

Reason:

1. General Trend: Across a period, electron gain enthalpy usually becomes more negative as we move left to right (due to increasing nuclear charge and smaller atomic size).
   → So, we might expect Fluorine to have the most negative value.
2. Exception in Fluorine:
   • Fluorine is exceptionally very small in size (atomic radius ≈ 42 pm).
   • Its valence shell (2p orbital) is very compact and already crowded with 7 electrons.
   • When an extra electron enters, it experiences very high electron–electron repulsions.
   • This reduces the stability gained after accepting the electron.
3. In Chlorine:
   • Chlorine has a larger size (3p orbital).
   • Repulsions between incoming electron and existing electrons are less.
   • The added electron is more easily accommodated, giving greater stability.

👉 Hence, Cl has more negative electron gain enthalpy than F.

✅ Conclusion: Order: Cl > F

(Chlorine has more negative electron gain enthalpy than Fluorine because F’s small size causes strong repulsions.)

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Why Chlorine (Cl) has a more negative electron gain enthalpy than Fluorine (F) ?

👉 This is a common JEE/NEET conceptual trap question because many students think F should have the maximum negative value.

📌 Electron Gain Enthalpy Values (approximate):

> Fluorine (F): –328 kJ/mol

> Chlorine (Cl): –349 kJ/mol

📊 Comparison:

❇️ Although F is smaller and more electronegative, its electron gain enthalpy is less negative than Cl due to very high electron–electron repulsion in its compact 2p orbital.

❇️ Cl has a more negative electron gain enthalpy because its larger 3p orbital can better accommodate the extra electron with lesser repulsion.

✅ Final Order: Cl (–349 kJ/mol) > F (–328 kJ/mol)

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