# Melting points of NaCl and RbCl

Why is the melting point of $\ce{NaCl}$ more than that of $\ce{RbCl}$ ? Using Fajan's Rule of polarizing power, $\ce{RbCl}$ has more ionic character and thus should have a higher melting point.

• See I am getting confused. Whether to use Fajan's rule or to go according to the periodic trend. One way we can say as the distance between ions increases bond strength decreases and hence melting point decreases. But then according to Fajan's rule the charge density over RbCl is least and hence covalent character would be minimum and hence melting point should be maximum. :/ Oct 28, 2015 at 5:17
• @Joel if we take into consideration of Molecular weight, then LiCl should have the highest melting point according to me. Oct 28, 2015 at 5:18
• @MaxW or do I just accept this also as an exception? Oct 28, 2015 at 5:19
• No. Think about it. Are the NaCl and RbCl "molecules" floating around in empty space?
– MaxW
Oct 28, 2015 at 5:23
• there would be anIonic bond. Oct 28, 2015 at 8:08

No doubt about it: it is due to the ionic radii and thus the distance between ions. The Rb-Cl bond is not as strong because of the distance between them. Na has an E.N. value of 0.9 Paulings while Rb is at 0.8; thus no significant difference in the magnetic forces of attraction.

LiCl has an even lower melting point due to excessive polarization (this is when Farjan's rules come into play), and the result, along with electronegativity again, is still an ionic compound, but with more covalent character.

• @Nicholas If you did use Fajan's rule for Li why not for Rb? Oct 29, 2015 at 11:03
• Well - look at the rule. Farjan states that small cation size is a contributor to polarization. Oct 29, 2015 at 19:37

See as you know they are ionic compounds and have some force of attraction. To break this energy is needed and the attraction between Na and Cl is more compared to Rb and Cl. Think about it like breaking a box when it is arranged in a lattice structure.