Question

why the melting point decreases from CaF2 to CaI2?

Answer 1

Melting point is directly proportinal to lattice energy.

Two main factors that contribute to the lattice energy of an ionic solid:

The charge on the ions, and the radius, or size, of the ions.
The effect of those factors is:
as the charge of the ions increases, the lattice energy increases.
as the size of the ions increases, the lattice energy decreases.


In this case, the size of anion goes on increasing from a small fluoride to a large iodide, hence the lattice energy decreases which in turn is the reason for the decrease in melting point.