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  4. Which one of the following arrangements does not give the correct picture of the trends indicated against it?

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Q. Which one of the following arrangements does not give the correct picture of the trends indicated against it?

AIPMTAIPMT 2008Classification of Elements and Periodicity in Properties

Solution:

Generally, bond dissociation energy follows the electronegativity trend. But in the case of halogens, a small change is there.
Fluorine is more electronegative than Chlorine. But Fluorine is very small compared to Chlorine. The bond length of Fluorine is less than Chlorine. There are more repulsions between Fluorine atoms in $F _{2}$ because of the abovementioned reasons.
If repulsions are there bond dissociation energy decreases. But in Chlorine, it doesn't happen. It has enough size and repulsions are less so the order of bond energy is:

$Cl _{2} > Br _{2} > F _{2} > I _{2}$