Which of the following compounds has the lowest melting point?
- CaCl2
- CaBr2
- CaI2
- CaF2
Answer
(3) CaI2
Solution
All four are halides of the same cation Ca2+; only the size of the anion changes:
F- < Cl- < Br- < I- (size)
A larger anion is more easily polarised by Ca2+ (Fajans' rule), the covalent character increases and the lattice energy decreases in the order:
CaF2 < CaCl2 < CaBr2 < CaI2 (covalent character)
Melting point: CaF2 > CaCl2 > CaBr2 > CaI2
CaI2, the most covalent of the four, has the lowest melting point.
Answer: (3)