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Lattice energy help?

NaI and KBr! Lattice energy?!?

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    U(NaI) = 682; U(KBr) = 671 kJ mol^-1 (+ve value used) CRC 2012 Handbook 12-23 From the Born-Lande Eqn (Wikipedia) U the lattice energy is proportional to Z+×Z-/(r+ + r-) everything else is irrelevant. Z+×Z- = 1 for both NaI and KBr. We .: conclude that the larger lattice energy of NaI is because the interionic distance is slightly shorter in NaI than KBr. Some radii: Na^+ = 116; K^= 152 pm e.g., http://www.webelements.com/sodium/atom_sizes.html Br^- 182; I^- 206 pm e.g., http://www.webelements.com/iodine/atom_sizes.html. I'll leave you to do the rest. Of course it's chemistry and there are exceptions to the rule and Li^+ salts don't obey the rule! Some argue that it is because Li^+ salts have covalent character but that should increase the lattice energy. I like the Pauling view that the Li^+ ion is exceptionally small (1s^2) and can't stabilize the lattice with cation touching anion without anion-anion repulsions..........

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U(NaI) = 682; U(KBr) = 671 kJ mol^-1 (+ve value used) CRC 2012 Handbook 12-23 From the Born-Lande Eqn (Wikipedia) U the lattice energy is proportional to Z+×Z-/(r+ + r-) everything else is irrelevant. Z+×Z- = 1 for both NaI and KBr. We .: conclude that the larger lattice energy of NaI is because the interionic distance is slightly shorter in NaI than KBr. Some radii: Na^+ = 116; K^= 152 pm e.g., http://www.webelements.com/sodium/atom_sizes.html Br^- 182; I^- 206 pm e.g., http://www.webelements.com/iodine/atom_sizes.html. I'll leave you to do the rest. Of course it's chemistry and there are exceptions to the rule and Li^+ salts don't obey the rule! Some argue that it is because Li^+ salts have covalent character but that should increase the lattice energy. I like the Pauling view that the Li^+ ion is exceptionally small (1s^2) and can't stabilize the lattice with cation touching anion without anion-anion repulsions..........

Mrs Biggles

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