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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.

Shailesh
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Adit Daftary
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  • 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. :/ – Adit Daftary Oct 28 '15 at 05:17
  • @Joel if we take into consideration of Molecular weight, then LiCl should have the highest melting point according to me. – Adit Daftary Oct 28 '15 at 05:18
  • @MaxW or do I just accept this also as an exception? – Adit Daftary Oct 28 '15 at 05:19
  • No. Think about it. Are the NaCl and RbCl "molecules" floating around in empty space? – MaxW Oct 28 '15 at 05:23
  • Another hint. Given that Na+, Rb+ and Cl- ions all have a different size, what would be the effect of that? – MaxW Oct 28 '15 at 05:36
  • I'm not sure here either just throwing it out there, would where the ions are on the scale of reactivity effect how strongly the ions are attracted which could be a melting point variable? Sodium is considered more reactive than lithium. – Technetium Oct 28 '15 at 05:39
  • Not reactivity. Just think of the Na+, Rb+ and Cl- ions as solid charged spheres - like individual legos. How do I get a solid? – MaxW Oct 28 '15 at 06:02
  • Yeah but how reactive they are would contribute to the bond strength and enthalpy? – Technetium Oct 28 '15 at 06:06
  • @MaxW I get the answer by what you have said. But then it disproves this Fajan's rule, doesn't it? – Adit Daftary Oct 28 '15 at 07:50
  • @Adit-Daftary - You can't possibly understand my point if you're still asking about Fajan's rule. I explicitly said to consider that the ions where single charged spheres. What kind of bond would be between an atoms with a + and - charges?? – MaxW Oct 28 '15 at 07:56
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    there would be anIonic bond. – Adit Daftary Oct 28 '15 at 08:08
  • The enthalpy of sublimination of the different compounds could be a factor here. Na has a higher enthalpy of sublimination than Rb. Li has a higher enthalpy of sublimination than both Na and Rb but due to excessive polarization (Fajans' rules), displays high covalent characteristics resulting in a lower sublimination point. I think. .. – Technetium Oct 28 '15 at 10:19
  • The same may apply to Rb. – Technetium Oct 28 '15 at 10:33

2 Answers2

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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
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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.

http://www.google.co.in/imgres?imgurl=https://upload.wikimedia.org/wikipedia/commons/e/e9/Sodium-chloride-3D-ionic.png&imgrefurl=https://en.wikipedia.org/wiki/Lattice_energy&h=948&w=1000&tbnid=Gm4SV7MURH7-UM:&docid=bdbhKWpu1E4z3M&hl=en-in&ei=DtkwVvSZCIjH0gTV0a4g&tbm=isch&client=safari&ved=0CCAQMygDMANqFQoTCPTY_9-t5cgCFYijlAod1agLBA

Epicflora
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