A graph of potential energy versus inter-nuclear distance for two chlorine atoms is given below

Explanation:

The sketch of the curve for two Br atoms can be seen in the image attached below. From the image below, we can deduce the following:

On the periodic table, the Bromine (Br) atom is the fourth member in the halogen family group. The atomic size increases from top to bottom down the group, thus Br atom posses a larger atomic size than Cl atom. As a result of that, the bond length formation between the two Br atoms will be larger compared to that of the two Cl atoms.

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