1. Why is the electron gain enthalpy of Chlorine more negative than Fluorine?
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Correct Answer
Chlorine has a more negative electron gain enthalpy than Fluorine because Fluorine is extremely small, causing strong electron–electron repulsion in its compact 2p orbital.
Detailed Explanation
Electron gain enthalpy is the enthalpy change when an electron is added to an isolated gaseous atom:
X(g) + e− → X−(g)
For the halogens, we might expect fluorine to have the most negative value because fluorine has the highest electronegativity. However, electron gain enthalpy and electronegativity are different properties.
Fluorine has a very small atomic size. Its incoming electron must enter the compact 2p orbital. Because the orbital is very small, the electrons are crowded together and electron–electron repulsion becomes significant.
In chlorine, the incoming electron enters the larger 3p orbital. The electron has more space, so electron–electron repulsion is comparatively lower.
Therefore, the addition of an electron to chlorine releases more energy:
ΔegH(Cl) < ΔegH(F)
Since a more negative value represents greater energy release:
Cl has more negative electron gain enthalpy than F.
Electronic Configuration
Fluorine:
F: 1s² 2s² 2p⁵
The added electron enters the compact 2p orbital:
F + e− → F− : 2p⁶
Chlorine:
Cl: [Ne] 3s² 3p⁵
The added electron enters the relatively larger 3p orbital:
Cl + e− → Cl− : 3p⁶
Thus, the electron–electron repulsion is greater in the very compact 2p orbital of fluorine.
Important JEE / NEET Concept
Do not confuse these two facts:
Electronegativity: F > Cl
but
Electron gain enthalpy: Cl is more negative than F
The reason is that electronegativity measures the tendency of an atom in a chemical bond to attract the shared electron pair, whereas electron gain enthalpy concerns the energy change when an isolated gaseous atom accepts an electron.
Easy Memory Trick
Fluorine = very small → electron crowding → high repulsion
Chlorine = larger 3p orbital → less repulsion → more energy released
Chlorine = larger 3p orbital → less repulsion → more energy released
Cl > F in magnitude of electron gain enthalpy
Therefore:
ΔegH(Cl) is more negative than ΔegH(F)
30 Related MCQs with Solutions
1. Which halogen has the most negative electron gain enthalpy?
A. F
B. Cl
C. Br
D. I
Answer: B. Cl
2. Why does fluorine show unusually less negative electron gain enthalpy than chlorine?
A. Low nuclear charge
B. Very small atomic size and high electron–electron repulsion
C. Large 3p orbital
D. Low electronegativity
Answer: B
3. The incoming electron in fluorine enters:
A. 1s
B. 2p
C. 3p
D. 3d
Answer: B. 2p
4. The incoming electron in chlorine enters:
A. 2p
B. 3s
C. 3p
D. 4s
Answer: C. 3p
5. Which has higher electronegativity?
A. Cl
B. F
C. Br
D. I
Answer: B. F
6. Which has more negative electron gain enthalpy?
A. F
B. Cl
C. Both equal
D. I
Answer: B. Cl
7. The 2p orbital of fluorine is:
A. Larger than 3p
B. Smaller and more compact than 3p
C. Equal to 3p
D. Empty
Answer: B
8. Greater electron–electron repulsion in F causes:
A. More energy release on electron addition
B. Less favourable electron addition
C. Larger size
D. Lower nuclear charge
Answer: B
9. Which statement is correct?
A. Electronegativity and electron gain enthalpy are identical
B. F has higher electronegativity but Cl has more negative electron gain enthalpy
C. Cl has higher electronegativity than F
D. Both properties are unrelated to electrons
Answer: B
10. Chlorine has a larger orbital for the incoming electron. This results in:
A. Greater electron repulsion
B. Lower electron repulsion
C. No electron addition
D. Zero nuclear attraction
Answer: B
11. The electron gain enthalpy of a halogen is generally:
A. Positive
B. Negative
C. Always zero
D. Infinite
Answer: B
12. Which halogen has the highest tendency to attract shared electrons in a bond?
A. F
B. Cl
C. Br
D. I
Answer: A. F
13. Which orbital experiences greater crowding in this comparison?
A. Cl 3p
B. F 2p
C. Both equal
D. Cl 4s
Answer: B. F 2p
14. The configuration of F before gaining an electron is:
A. 1s²2s²2p⁴
B. 1s²2s²2p⁵
C. 1s²2s²2p⁶
D. 1s²2s²3p⁵
Answer: B
15. The configuration of Cl before gaining an electron is:
A. [Ne]3s²3p⁵
B. [Ne]3s²3p⁶
C. [Ar]3s²
D. [He]2s²2p⁵
Answer: A
16. Which factor explains the anomaly in electron gain enthalpy of F?
A. Very large size
B. Compact 2p orbital
C. Empty d orbitals
D. Low nuclear charge
Answer: B
17. Which species is formed when Cl gains one electron?
A. Cl+
B. Cl−
C. Cl²⁺
D. Cl²⁻
Answer: B. Cl⁻
18. Which species is formed when F gains one electron?
A. F+
B. F−
C. F²⁺
D. F²⁻
Answer: B. F⁻
19. The greater negativity of Cl electron gain enthalpy means:
A. More energy is released on electron addition
B. More energy is absorbed
C. No energy change
D. Electron cannot be added
Answer: A
20. Which comparison is correct?
A. F 2p > Cl 3p in size
B. Cl 3p > F 2p in size
C. Both equal
D. 2p is empty
Answer: B
21. Which property is measured for an isolated gaseous atom?
A. Electron gain enthalpy
B. Bond angle
C. Hybridisation only
D. Dipole moment only
Answer: A
22. Why does atomic size affect electron gain enthalpy?
A. It affects electron–electron repulsion and attraction
B. It changes the number of protons
C. It removes the nucleus
D. It changes the element
Answer: A
23. Which statement about fluorine is true?
A. It has the largest atomic radius among halogens
B. It has the highest electronegativity
C. It has the most negative electron gain enthalpy
D. It has no valence electrons
Answer: B
24. Which statement about chlorine is true?
A. Its electron gain enthalpy is more negative than F
B. It is more electronegative than F
C. It has a smaller 2p orbital
D. It has no p electrons
Answer: A
25. The unusual order of F and Cl electron gain enthalpies is mainly due to:
A. Nuclear instability
B. Small size of F and high repulsion in 2p
C. Large size of F
D. Empty 3p orbital of Cl
Answer: B
26. The electronic configuration of Cl⁻ is:
A. [Ne]3s²3p⁵
B. [Ne]3s²3p⁶
C. [Ar]3s¹
D. [Ne]3p⁶
Answer: B
27. The electronic configuration of F⁻ is:
A. [He]2s²2p⁵
B. [Ne]
C. [He]2s²2p⁶
D. [Ar]
Answer: B. [Ne]
28. Which trend is generally observed in electron gain enthalpy down Group 17, with the F anomaly noted?
A. It becomes progressively more negative without exception
B. It generally becomes less negative down the group
C. It becomes zero for all elements
D. It becomes positive for all elements
Answer: B
29. Which factor is NOT responsible for the unusual F–Cl order?
A. Compact 2p orbital of F
B. Electron–electron repulsion
C. Larger 3p orbital of Cl
D. Fluorine having lower electronegativity than chlorine
Answer: D
30. The best summary is:
A. F is most electronegative and also has the most negative electron gain enthalpy
B. F is most electronegative, but Cl has more negative electron gain enthalpy
C. Cl is more electronegative and has more negative electron gain enthalpy
D. Both properties are identical
Answer: B
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