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Everything else the same, I'd expect two monoatomic ions to form an ionic structure in the CsCl structure because with more atoms bonded to each atom, it would seem to be more stable. And yet I believe the alkali halides tend to form in the NaCl structure more often -- and even CsCl will reform into the NaCl structure if heated high enough.

In the simplest ionic compounds (monoatomic), what are the main reasons why some form in one structure vs the other?

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The structure in these cases is determined by the relative sizes of the ions. Caesium and chloride have very similar ionic radii (174 and 181 pm, respectively), whereas sodium and chloride are very different (102 and 181 pm).

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  • $\begingroup$ So the reason is that there isn't enough room in compounds with 2 very differently sized ions to pack them in tightly with the CsCl structure? And so because bond strength is inversely proportional to distance, it is more favorable to pack into a structure with less bonds per atom? $\endgroup$ Sep 20, 2014 at 1:34
  • $\begingroup$ @user1833709 That's right. $\endgroup$
    – lemon
    Sep 20, 2014 at 14:49

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