I'd think that increasing ionic character by increasing electronegativity difference would result in strong bonds but in the case of NaCl and CsCl(and the other chloride of the group) the case is different.

  • $\begingroup$ Related: chemistry.stackexchange.com/questions/82246/… $\endgroup$ Sep 17 '17 at 9:09
  • $\begingroup$ Ionic compounds have no bonds. It's a question of efficient packing and electrostatics. $\endgroup$
    – Karl
    Sep 17 '17 at 9:34
  • 2
    $\begingroup$ @Karl Ionic compounds have bonds, just constructed differently from those th things like water and methane. $\endgroup$ Sep 17 '17 at 9:46
  • $\begingroup$ @OscarLanzi A "bond" implies directional connection. No such thing in sodium chloride. The term is simply not helpful in explaining the properties of ionic compounds, as can be see by the above question, where it has mislead the OP. $\endgroup$
    – Karl
    Sep 17 '17 at 10:49
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    $\begingroup$ @Karl A bond does not imply directionality. An ionic bond is a perfectly valid bond whether you are discussing the 'real' ionic bond (with non-zero covalent/metallic character), or the ideal 'pure' ionic bond. Since this is an issue of chemical definitions, I refer you to the IUPAC Gold Book. $\endgroup$ Sep 17 '17 at 11:45

Sodium chloride and caesium chloride are both primarily ionic compounds. Within ionic compounds, a greater melting point can be caused by a higher ionic charge or a higher charge density/better interaction. Since both have the same charge while sodium has a smaller ionic radius, the sodium-based crystal is held together better due to more interaction than the caesium based one.

When comparing barium chloride and beryllium chloride, you are essentially comparing apples and oranges since beryllium chloride is a primarily molecular compound. In its gas phase, it forms $\ce{BeCl2}$ monomers and $\ce{Be2Cl4}$ dimers. The intramolecular attractions in molecular compounds are typically rather strong with the intermolecular ones are weaker. For ionic compounds such as barium chloride, we cannot distinguish between the two since we cannot define molecular entities.

When comparing ionic compounds to molecules, the ionic compounds typically have a much higher melting point (with exceptions; both low-melting ionic compounds and high-melting molecular compounds).

  • $\begingroup$ Arright, that explains it for BeCl2 but what about CaCl2 (I don't think this dimerises, does it?) ? It's smaller but has a lower melting point it looks like (does CaCl2 even exist in a crystal lattice?) $\endgroup$
    – Vrisk
    Sep 17 '17 at 13:50
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    $\begingroup$ Well a quick google suggests they have different crystal structures, CaCl2 taking a rutile structure, and BaCl2 being calcite or PbCl2. It's not easy to compare different structures, especially when coordination numbers change as in this case, so periodic trends such as MP will be hard to spot. In fact the most obvious periodic trend in the alkaline earth chlorides is the change from covalent to ionic behaviour, from covalent BeCl2 through the layered MgCl2 to the typical ionic structure of BaCl2. $\endgroup$
    – Ian Bush
    Sep 17 '17 at 14:44

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