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Why is the lattice energy of Nacl more than that of CsCl? If size of Na is smaller than that of Cs, then according to Fajan's rule, Na should polarise Chlorine more and therefore, NaCl must have more covalent character than CsCl (and higher covalent character means low lattice energy).

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    $\begingroup$ CsCl is very much an ionic compound, and NaCl (if Fajan's rule applies) is a tiny bit more covalent. Makes no difference. $\endgroup$ – Karl Oct 21 '18 at 14:14
  • $\begingroup$ Diamond has quite a high covalent character, but I believe its lattice energy is quite high ... $\endgroup$ – Ian Bush Oct 21 '18 at 21:33
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Lattice Energy is directly proportional to the Charge on ion and inversely proportional to radius of atom . Now in between NaCl and CsCl the radius of Cs+ is way more than Na+ , even though CsCl is more ionic but still radius is more and hence the lattice energy is less .

It can also be explained using

Kapustinskii Equation

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