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Fajans' rules
Concepts tested here
- Covalent character in ionic compounds
All Questions
2011 AIPMT-PRE 1 question
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Which of the following compounds has the lowest melting point?All four compounds have the same cation, $Ca^{2+}$; only the anion changes.
By Fajans' rules, the larger the anion, the more easily it is polarised by the cation and the greater the covalent character of the compound.
Anion size: $F^- < Cl^- < Br^- < I^-$, so covalent character: $CaF_2 < CaCl_2 < CaBr_2 < CaI_2$.
Also, the lattice energy decreases as the anion becomes larger.
Greater covalent character and lower lattice energy mean a lower melting point.
Hence $CaI_2$ has the lowest melting point.