Question

On the basis of strength of intermolecular forces, which of the following elements would be expected to have the highest melting point?

On the basis of strength of intermolecular forces, which of the following elements would be expected to have the highest melting point?
(A)`Br_2`
(B) `Cl_2`
(C) `F_2`
(D) `Kr`
(E) `N_2`

I understand that LDF strength increases with increasing charge and increasing molecular size. Based on that, would not `Kr` has the strongest LDF and hence the highest melting point? Why is `N_2` the correct answer?

Answer 1

(A) `Br_2` has the highest melting point.

Explanation:

You're right in that `Kr` has a greater degree of intermolecular forces than `N_2`!
They're both nonpolar molecules, and `Kr` has a greater number of polarizable electrons (`36` electrons), so `Kr` would have a greater degree of LDFs and therefore a greater melting point than `N_2` (which has `7xx2=14` polarizable electrons).

Note: Molecules with a greater number of polarizable electrons will have a greater degree of LDFs, because more electrons will both increase the chance of forming an instantaneous dipole and increase the polarity of that instantaneous dipole.

But we can't forget about our other choices:
(Everything in this question is nonpolar, so we can safely base our judgments of intermolecular forces on the number of polarizable electrons):

• `Br_2`, which has `35 xx 2 = 70` polarizable electrons.
• `Cl_2`, which has `17 xx 2 = 34` polarizable electrons.
• And `F_2`, which has `9xx2=18` polarizable electrons.

In the end, we find that `Br_2` has the most number of polarizable electrons of them all.
That's why it has the greatest degree of intermolecular forces.