Among LiH, NaH, KH, RbH, CsH the one with the highest melting point is LiH, and it keeps on decreasing till CsH. The reason for this in my textbook is that the size of cations keeps on increasing down a group, hence the lattice energy keeps on decreasing, hence the mp. and bp. order.

But when I apply Fajans rules, as we go down a group, the size of cation increases as we go down the group, hence the polarisation power of cation decreases, hence the covalent character decreases, so the ionic character increases, so the melting point and the boiling point should increase down the group.

Where exactly am I going wrong?



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