KCET · Chemistry · p Block Elements (Group 15, 16, 17 & 18)

The property of halogens which is not correctly matched is

A \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (Electronegativity)
B \(\mathrm{I}>\mathrm{Br}>\mathrm{Cl}>\mathrm{F}\) (Density)
C \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (Electron gain enthalpy)
D \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (Ionisation enthalpy)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}\) (Electron gain enthalpy)

Step-by-step Solution

Detailed explanation

Down the group, electron gain enthalpies become less and less negative with the increase in the size of the halogen. However, electron gain enthalpy of \(\mathrm{F}\) is less negative than \(\mathrm{Cl}\). This is due to small size of F-atom. As a result, there are strong inter electronic repulsions in the relatively small \(2 p\)-orbitals of \(\mathrm{F}\) and thus incoming electrons experience much less attraction. Thus, the correct order is \(\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}\).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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