[navalava]

Chlorine is more electronegative than bromine, which would make bonds more polar, which would have a greater effect on bond strength than simply increased london dispersion forces (which is all that Bromine would offer with its increased number of electrons).

[orgopete]

If only that were true. If I said you had to explain your answer based upon the four forces of nature (the strong force, the weak force, gravity, or electromagnetic force), then you would have to explain it based upon some electromagnetic effect. Hydrogen bonding or dipole-dipole interactions are the result of electromagnetic forces.

Let me give you some data. The boiling point of alkyl bromides are higher than chlorides and in turn higher than fluorides. If you had to explain this by offering forces based upon Coulomb's Law, what would you suggest? Would you expect your prediction to match other data, e.g., reaction rates? If you compare your explanation with the rate of reaction with a nucleophile, do they match? Does the explanation suggested by electronegativity match the data obtained by bp and reaction rates? Remember, electronegativity was suggested to explain why some atoms had high homolytic bond strengths.

You might also look at the discussion here:
http://www.chemicalforums.com/index.php?topic=45100.5

[Guitarmaniac86]

Think about the number of electrons in bromine compared to chlorine and how this will effect inter molecular bonding.