Why is the EGE of Chlorine more negative than Fluorine?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Answer

The electron gain enthalpy (EGE) of chlorine is more negative than that of fluorine because the incoming electron enters the larger 3p orbital of chlorine, where electron-electron repulsion is less than in the very small 2p orbital of fluorine.

Cl: [Ne] 3s² 3p⁵

F: [He] 2s² 2p⁵

Although fluorine has a higher effective nuclear charge, its very small atomic size causes considerable repulsion between the incoming electron and the electrons already present in the compact 2p subshell.

⚛️ Comparison

F → Small 2p orbital → High electron-electron repulsion

Cl → Larger 3p orbital → Less electron-electron repulsion

Therefore, chlorine can accommodate the incoming electron more easily than fluorine, releasing more energy.

EGE(Cl) < EGE(F)

Hence, the EGE of chlorine is more negative than that of fluorine.

🔹 Important Exam Point

The expected trend of electron gain enthalpy in Group 17 is not perfectly regular. Fluorine has a less negative EGE than chlorine because of its extremely small size and greater electron-electron repulsion in the compact 2p subshell.

Cl has the most negative electron gain enthalpy among the halogens.

✅ Final Answer

Chlorine has a more negative EGE than fluorine because its incoming electron enters the larger 3p orbital, where electron-electron repulsion is lower. In fluorine, the incoming electron enters the very small 2p orbital, causing greater repulsion.

EGE(Cl) < EGE(F)

20 MCQs with Answers

1. Which has more negative electron gain enthalpy?

A) Fluorine

B) Chlorine

C) Oxygen

D) Nitrogen

✅ Answer

B) Chlorine

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2. The incoming electron in fluorine enters the:

A) 1s orbital

B) 2p orbital

C) 3p orbital

D) 3d orbital

✅ Answer

B) 2p orbital

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3. The incoming electron in chlorine enters the:

A) 2p orbital

B) 3s orbital

C) 3p orbital

D) 4s orbital

✅ Answer

C) 3p orbital

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4. Why is EGE of F less negative than Cl?

A) F has no nucleus

B) F has a very small 2p orbital

C) Cl has fewer electrons

D) F is a metal

✅ Answer

B

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5. Greater electron-electron repulsion occurs in:

A) Large 3p orbital

B) Compact 2p orbital

C) 4s orbital only

D) 3d orbital only

✅ Answer

B

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6. Fluorine belongs to:

A) Group 1

B) Group 2

C) Group 17

D) Group 18

✅ Answer

C) Group 17

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7. Chlorine belongs to:

A) Group 15

B) Group 16

C) Group 17

D) Group 18

✅ Answer

C) Group 17

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8. Chlorine has the configuration:

A) [He] 2s²2p⁵

B) [Ne] 3s²3p⁵

C) [Ne] 3s²3p⁶

D) [Ar] 4s¹

✅ Answer

B

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9. Fluorine has the configuration:

A) [He] 2s²2p⁵

B) [Ne] 3s²3p⁵

C) [He] 2s²2p⁴

D) [Ne] 3s²3p⁶

✅ Answer

A

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10. Which factor makes electron addition to F relatively difficult?

A) Very large size

B) Compact size and electron repulsion

C) Absence of electrons

D) Low nuclear charge

✅ Answer

B

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11. Electron gain enthalpy is generally represented in:

A) kJ mol⁻¹

B) kg mol⁻¹

C) metre

D) litre

✅ Answer

A

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12. A more negative EGE indicates:

A) Greater tendency to accept an electron

B) Greater tendency to lose a proton

C) No electron affinity

D) No nuclear attraction

✅ Answer

A

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13. Which subshell is smaller?

A) 3p

B) 2p

C) Both are equal

D) 4p

✅ Answer

B) 2p

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14. The atomic number of fluorine is:

A) 7

B) 8

C) 9

D) 10

✅ Answer

C) 9

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15. The atomic number of chlorine is:

A) 16

B) 17

C) 18

D) 19

✅ Answer

B) 17

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16. The EGE anomaly between F and Cl is mainly due to:

A) Nuclear instability

B) Small size of F

C) Large mass of F

D) Metallic character of F

✅ Answer

B

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17. Which halogen has the most negative EGE?

A) F

B) Cl

C) Br

D) I

✅ Answer

B) Cl

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18. Fluorine gains one electron to form:

A) F⁺

B) F²⁺

C) F⁻

D) F²⁻

✅ Answer

C) F⁻

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19. Chlorine gains one electron to form:

A) Cl⁺

B) Cl⁻

C) Cl²⁺

D) Cl²⁻

✅ Answer

B) Cl⁻

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20. The main reason for the higher EGE magnitude of Cl compared with F is:

A) Larger 3p orbital reduces electron-electron repulsion

B) Chlorine has no valence electrons

C) Fluorine has no nucleus

D) Chlorine is a noble gas

✅ Answer

A

Q. Why is the electron gain enthalpy of chlorine more negative than fluorine?

Solution: Fluorine is very small, so the incoming electron experiences strong electron-electron repulsion in its compact 2p orbital. In chlorine, the larger 3p orbital provides more space and less repulsion.

Therefore, EGE(Cl) is more negative than EGE(F).

Q. Explain why chlorine has a more negative electron gain enthalpy than fluorine.

Solution:

• Fluorine has a very small atomic size and a compact 2p subshell.
• The incoming electron experiences strong electron-electron repulsion.
• Chlorine has a larger 3p subshell, so electron-electron repulsion is comparatively lower.

Hence, chlorine releases more energy on gaining an electron and its EGE is more negative.

Q. Compare the electron gain enthalpy of fluorine and chlorine and explain the anomaly in Group 17.

Solution:

Normally, electron gain enthalpy is expected to become less negative down a group because atomic size increases. However, fluorine is an exception.

F: Very small 2p orbital → High repulsion

Cl: Larger 3p orbital → Lower repulsion

Thus, the incoming electron is accommodated more comfortably in chlorine than in fluorine.

EGE(Cl) < EGE(F)

Chlorine has the more negative electron gain enthalpy.

Q. Explain the electron gain enthalpy trend in halogens, with special reference to fluorine and chlorine.

Solution:

1. Halogens have seven valence electrons and require one electron to attain a stable noble-gas configuration.
2. Therefore, they generally have highly negative electron gain enthalpies.
3. Fluorine has a very small atomic size and compact 2p orbitals.
4. The incoming electron experiences strong electron-electron repulsion in fluorine.
5. Chlorine has a larger 3p orbital, allowing the incoming electron to be accommodated with less repulsion.

Cl has more negative EGE than F.

Q. Explain in detail why the electron gain enthalpy of chlorine is more negative than fluorine.

Detailed Solution

Both fluorine and chlorine belong to Group 17 and have seven valence electrons. They need one electron to complete their valence shell. Therefore, both have a strong tendency to accept an electron.

Electronic Configurations

F = 1s² 2s² 2p⁵

Cl = [Ne] 3s² 3p⁵

Fluorine is much smaller than chlorine. The incoming electron in fluorine has to enter the compact 2p orbital. Because the orbital is very small, there is considerable electron-electron repulsion between the incoming electron and the electrons already present.

In chlorine, the incoming electron enters the larger 3p orbital. The greater size of the orbital provides more space and reduces electron-electron repulsion.

Small 2p orbital (F)
↓
Greater repulsion

Larger 3p orbital (Cl)
↓
Less repulsion

Hence, chlorine releases more energy when it gains an electron. Therefore, its electron gain enthalpy is more negative than that of fluorine.

Final Answer:

EGE(Cl) < EGE(F)

The unusual trend is mainly due to the very small size of fluorine and the high electron-electron repulsion in its compact 2p subshell.

🎯 Quick Revision

F: Small 2p orbital → More repulsion
Cl: Larger 3p orbital → Less repulsion

EGE(Cl) < EGE(F)

Exam Point: Chlorine has the most negative electron gain enthalpy among the halogens.