Answer

Cl2 and Br2 have approximately the same shape and neither is polar.

            a. Upon cooling, both Cl2 and Br2 form solids.  Why?
            b. At 25oC, chlorine (Cl2) is a gas whereas bromine (Br2) is a liquid. Why?

  1. London dispersion forces are responsible for the formation of the solids.  As the elements are cooled, the kinetic energy of the Cl2 and Br2 molecules decreases and the London forces are strong enough to overcome the kinetic energy and hold the molecules in a solid.

  2. London dispersion forces between the larger Br2 molecules are sufficient to cause them to form a liquid at 25oC, whereas London dispersion forces between the smaller Cl2 molecules are not.
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