Melting (and boiling) temperatures

 

The discussion on relative bond strengths would suggest that covalent compounds have the highest melting points and metals the lowest. Stronger bonds require more energy in the form of heat to break them. Indeed, typically ionic compounds do have higher melting points than metals. This is why brick ovens are used in metal smelting. But covalent compounds seem to have missed their calling completely. Most gases and liquids in our experience are covalently bonded. At room temperature they have already not only melted, but the gases have boiled. Other commonly encountered covalent compounds include candles, oils and plastics of various descriptions. What gives?

Crystals of molecules, sugar, for example, are composed of many molecules. The important difference is that within a crystal of a metal ALL the bonds are metallic, within a crystal of an ionic solid ALL the bonds are ionic between the ions and within a crystal of a covalent network ALL the bonds are covalent. Within a crystal of a molecule the bonds within the molecules are covalent, but the bonds between molecules are not. The forces that hold one molecule to the next in order to build up a crystal, don’t even get called bonds. They are usually referred to just as forces.There are several of these forces, but the distinctions between them do not concern us here. For this discussion, the important thing to get hold of is that they are all far weaker than any of the bonds we have been discussing.When ice melts, nothing is happening to the covalent H-O bonds. The change is happening between one water molecule and the next. Bonds are not being disturbed, only much weaker intermolecular forces.

Taking that into consideration, you want to recall that the covalent networks do indeed have very high melting points. Sand, diamond and graphite all belong to this group.

The terms used for covalent forces and bonds are intramolecular bonds - the actual shared pairs of electrons, and intermolecular forces BETWEEN molecules. If you find the two terms confusing, think of intrastate football as opposed to interstate football, one is within a state, the other between states.
 

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