Question:
Which element from following has positive electron gain enthalpy?
Which element from following has positive electron gain enthalpy?
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Noble gases have a full octet, so adding an electron needs energy.
Updated On: Oct 1, 2026
- Xenon
- Bromine
- Iodine
- Chlorine
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Concept:
Electron gain enthalpy is the energy change when a neutral gaseous atom takes up an electron. Most nonmetals release energy, so the value is negative. A positive value means energy must be supplied.
Step 2: Key Approach:
An atom with a completely filled valence shell has no vacant stable place for the new electron. The incoming electron must go into the next higher shell, which costs energy.
Step 3: Detailed Explanation:
Xenon is a noble gas with the configuration \([\text{Kr}]\,4d^{10}5s^25p^6\), a complete octet in its outermost shell. An extra electron would have to enter the \(6s\) level. This is unfavourable, so energy is absorbed and the electron gain enthalpy of Xenon is positive.
Halogens such as Cl, Br and I are one electron short of an octet. They gain an electron easily and release energy.
Step 4: Why the other options are wrong.
Bromine (B), iodine (C) and chlorine (D) all have large negative electron gain enthalpies. Chlorine has the most negative value in the periodic table, about \(-349\) kJ/mol.
Final Answer:
Only the noble gas Xenon has a positive electron gain enthalpy, option (A). \[ \boxed{\text{Xenon (A)}} \]
Electron gain enthalpy is the energy change when a neutral gaseous atom takes up an electron. Most nonmetals release energy, so the value is negative. A positive value means energy must be supplied.
Step 2: Key Approach:
An atom with a completely filled valence shell has no vacant stable place for the new electron. The incoming electron must go into the next higher shell, which costs energy.
Step 3: Detailed Explanation:
Xenon is a noble gas with the configuration \([\text{Kr}]\,4d^{10}5s^25p^6\), a complete octet in its outermost shell. An extra electron would have to enter the \(6s\) level. This is unfavourable, so energy is absorbed and the electron gain enthalpy of Xenon is positive.
Halogens such as Cl, Br and I are one electron short of an octet. They gain an electron easily and release energy.
Step 4: Why the other options are wrong.
Bromine (B), iodine (C) and chlorine (D) all have large negative electron gain enthalpies. Chlorine has the most negative value in the periodic table, about \(-349\) kJ/mol.
Final Answer:
Only the noble gas Xenon has a positive electron gain enthalpy, option (A). \[ \boxed{\text{Xenon (A)}} \]
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