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We have to compare melting points of $\ce{LiH}$, $\ce{NaH}$, $\ce{KH}$, $\ce{CsH}$. I know the melting point of an ionic compound is more than that of a covalent compound.

So according to Fajan's rule, ionic character increases in $\ce{LiH}$, $\ce{NaH}$, $\ce{KH}$, $\ce{CsH}$. As ionic character increases, melting point should also increase in $\ce{LiH}$, $\ce{NaH}$, $\ce{KH}$, $\ce{CsH}$.

But actually, the melting point decreases in $\ce{LiH}$, $\ce{NaH}$, $\ce{KH}$, $\ce{CsH}$. What could be the reason behind this?

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  • $\begingroup$ Why do you think that the ionic character is the factor that matters the most? $\endgroup$ – Zhe Mar 30 '17 at 18:20
  • $\begingroup$ @Zhe because to compare melting point we have to compare bond strength . And I think ionic bond has strongest bond strength. $\endgroup$ – search Mar 30 '17 at 18:35
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    $\begingroup$ So, compare the melting temperature of diamond or tungsten vs these compounds. Note that labeling a bond as ionic, covalent, or metallic says very little about how 'strong' they are. $\endgroup$ – Jon Custer Mar 30 '17 at 18:58
  • $\begingroup$ Okay , I got my misconception . @JonCuster then here what could be the reason . $\endgroup$ – search Mar 30 '17 at 19:03
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    $\begingroup$ Melting point is a somewhat tricky concept because it heavily depends on the structure of the bulk solid. Are you sure that the crystal packing in all 4 substances is the same? If not, then you are comparing apples to oranges. $\endgroup$ – Zhe Mar 30 '17 at 19:43
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You can get an estimate of the bond strength by using Coulomb's law and periodic trends for atomic radius. $$F = k{ {q_1q_2} \over r^2}$$The charge of all these alkali metals is the same in these compounds, so the only difference in bond strength comes from the the distance between the charges. The distance between the charges is related to each atoms' atomic radius and that is an increasing trend as you move down the alkali metals.

So, if distance is increasing, the force holding the solid together is decreasing, which would justify the trend $\ce{LiH}>\ce{NaH} >\ce{KH}> \ce{CsH}$ for melting point.

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