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Arrange the following in the increasing order of melting point of different types of crystalline solids

  1. covalent solid
  2. metallic solid
  3. molecular
  4. ionic

According to me the arrangement should be:

ionic > covalent > metallic > molecular

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3 Answers 3

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When solids are held together on a macroscopic scale by covalent, ionic or metallic bonds they are more or less equally bonded and thus equally likely to have high melting points. How high depends on structural features that do not directly correlate with whether the bonding is covalent, ionic or metallic.

We illustrate by comparing the melting points of some common materials. Magnesia melts at a higher temperature than silica not because the former is ionic and the latter covalent, but because magnesia forms a crystal structure with higher coordination numbers and thus more bonds to break than silica. Sodium chloride firms a crystal structure similar to magnesia but sodium chloride involves fewer electrons in the bonding and has larger chlorine atoms taking up most of the crystal space instead of smaller oxygen atoms. The combination of fewer electrons and greater distances for the electrostatic bonding interactions renders sodium chloride lower-melting than either magnesia or silica.

As these examples demonstrate, melting points of substances with macroscopically extensive "primary" bonds involve complex combinations of factors that go well beyond the type of bond formally assigned to them.

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No universal ordering is possible as there are too many exceptions

It can certainly be argued that there is a broad ordering of melting point based on the type of bonding in a solid. But there are far too many exceptions for the ordering to be reliable.

And the question contains a subtle distinction that it doesn't explicitly call out: the difference between a molecular solid and a covalent solid. It also uses what I would call careless language: what is the difference between a crystalline solid and the other categories (all of which can form well defined crystals). (Note: If the original question said ionic not crystalline my interpretation is unfair.)

Some specific examples that illustrate the problems. There are ionic compounds that are liquids not crystalline solids, though many ionic compounds are, like table salt, high melting. There are several metals that are soft or melt easily (gold, sodium), some that are liquids (mercury, caesium) some that are strong and melt at high temperatures (tungsten, rhenium). Some molecular solids have high melting points (graphite, silica) and some that have much lower melting points (some glasses).

The only group that usually has low melting points are molecular solids where the forces that matter are typically weak intermolecular forces. Hence why things like menthol are volatile, low-melting crystals.

Given some of those observations, the type of bonding is not a good universal predictor of the properties of the compound.

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    $\begingroup$ Dunno why you bothered with answer such a question, but if you already did... NaCl has a lower melting point than gold; graphite and silica are covalent, but without discrete molecules; caesium is borderline, galinstan is a clear cut example of liquid metal; molecular solids can be pretty high melting... $\endgroup$
    – Mithoron
    Apr 3, 2023 at 0:58
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I am not sure such a general classification can be made. Just for fun I would probably put both ionic and metallic solids together in the first place, followed by covalent compounds. But there are exceptions, like the mercury $\ce{Hg}$ (metal, melting at $\pu{-39 °C}),$ and the covalent silica $\ce{SiO2}$ melting at $\pu{1730 °C}.$

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