Why does the ionic compound calcium fluoride have a different structure from that of calcium chloride?

Answer 1

Calcium fluoride has the fluorite structure where each calxium ion has 8 fluoride ions as near neighbours at the corners of a cube, and eacf fluoride has 4 calcium ions ate the corners of a tetrahedron. Another way of looking at this is that the calcium ions are close packed and the fluroide atoms fill the "tetrahedral holes"

In calcium chloride the chloride ion is too large to allow eight coordination and the coordination around the calcium drops to 6, the coordination around the chloride has to drop to 3. This gives CaCl2 a unique structure which is a distorted form of the more regular rutile structure adopted by TiO2.

Answer 2

The difference in structure between calcium fluoride and calcium chloride is due to the difference in the size and charge of the anions. Fluoride ions are smaller than chloride ions, leading to a tighter packing arrangement in calcium fluoride. Additionally, the charge of the ions influences the overall structure of the compound.