Why Is MgF2 the Hardest Among NaF, NaI, MgI2, and CaI2?

[Neophyte]

Homework Statement

Which of the following is expected to have the greatest hardness?
NaF
NaI
MgF2
MgI2
CaI2

The Attempt at a Solution

I can not seem to find what affects the hardness of them, only how to calculate it after you have the data from experiments via google. I know the answer is MgF2 I do not know how though :/. Any help as to what direction to look would be tremendously helpful.

 

[Borek]

Think in terms of coulomb forces - charges and distances.