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I read that to compare thermal stabilities of ionic solids:

if the solid has a monoatomic anion, we compare thermal stability on the basis of lattice energy which is inversely proportional to size.

if the solid has a polyatomic anion, we compare thermal stability on the basis of ionic character which can we done using Fajans' Rule.

Could someone please confirm that these are correct?

I have a doubt because I also read from somewhere else that MgCl2 is less thermally stable than BaCl2,

which contradicts the rule as Cl- is monoatomic, Ba2+ is larger in size, so BaCl2 has lesser lattice energy so it should be less thermally stable. So what is the correct answer?

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  • $\begingroup$ I also came across this order(without any proper sequence) for thermal stability of alkali metal chlorides: LiCl>KCl>NaCl>CsCl. What could be the possible reason? $\endgroup$ – thewitness Oct 9 '18 at 5:30
  • $\begingroup$ In this case, I think lattice energy should be the primary consideration. Magnesium chloride has the most exothermic LE while barium chloride has the least exothermic one. Thus, MgCl2 with the most exothermic LE, is the most thermally stable. $\endgroup$ – Tan Yong Boon Oct 10 '18 at 5:10
  • $\begingroup$ Isn't the lattice energy of MgCl2 more than that of BaCl2 as Mg2+ is smaller than Ba2+. I googled the values I got MgCl2 is +2326 KJ/mol and BaCl2 is +2056KJ/mol. $\endgroup$ – thewitness Oct 10 '18 at 6:05
  • $\begingroup$ Lattice energies are by definition exothermic. Where did you get your values from? $\endgroup$ – Tan Yong Boon Oct 10 '18 at 6:15
  • $\begingroup$ There are two definitions of lattice energy. One of formation other of disassociation(just opposite in sign but magnitude remains same). $\endgroup$ – thewitness Oct 10 '18 at 6:30

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