1. The problem statement, all variables and given/known data Which of the following is expected to have the greatest hardness? NaF NaI MgF2 MgI2 CaI2 3. 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.