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I get really confused by this:

The order of melting points of group 2 chlorides is: $$\ce{BaCl2 > SrCl2 > CaCl2 > MgCl2 > BeCl2}$$ This is completely understandable by Fajan's Rules.

But, the order for group 1 halides is: $$\ce{NaCl > KCl > RbCl > CsCl > FrCl > LiCl}$$

This is what I don't get! Considering Fajan's Rules, $\ce{NaCl}$ is the most covalent, least cationic size, so shouldn"t it have the lowest melting point?

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For group 1 halides polarisation is not a dominating factor(except for lithium) since the size of cations is relatively larger than any other cation therefore we must consider lattice energy for determining melting point. As interionic distance increases lattice energy decreases hence melting point decreases down the group. Lithium is an exception because of its very small size that brings high degree of polarisation. you may want to refer to lother meyer's curve on atomic volumes of elements to get relative idea about the size of the elements.

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Although lattice energy of LiCl higher than NaCl, but LiCl is covalent in nature and NaCl ionic. There after, the melting point decreases as we move NaCl because the lattice energy decreases as size of alkali metal atom increases. Higher lattice energy denotes higher melting and boiling points.

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Melting point depends on Lattice energy. NaCl has similar sized ions which make the lattice stronger. LiCl comes last since it has a lot of covalent character.

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  • $\begingroup$ Thanks for the answer. So we cant just predict their properties simply using fajans rules, isnt it? We need to know the trends experimentally, and then "explain" them somehow... $\endgroup$
    – M31
    Mar 27 '15 at 11:33
  • $\begingroup$ Fajan's rules doesnt say anything about melting point. Just about polarization. $\endgroup$ Mar 27 '15 at 11:36

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