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I am school student. In my school text book there is a fact about diamond. That is

Each carbon atom is covalently bonded to four other carbon atoms. A lot of energy is needed to separate the atoms in diamond. This is because covalent bonds are strong, and diamond contains many covalent bonds. This makes diamond's melting point and boiling point very high.

If a diamond is a single molecule, how does a single molecule melt? Is that fact correct? Please tell me the answer!

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The melting point of diamond is certainly an exotic topic. Diamond is slightly less stabile than its allotrope, graphite, and will in most circumstances turn to graphite way before its melting point. If you have enough pressure it will stay diamondlike and it will at some point melt.

A diamond is not a single molecule unless you stretch the definition of molecule. It is usually defined as the smallest fundamental unit, and for diamond I'd call the tetragonal carbon unit cell the smallest fundamental unit, or maybe even a carbon atom. Diamond, having one of the highest values for heat transfer among substances would certainly melt almost like a single molecule though. But in theory, one and one carbon decides to leave behind it's friends bound by the diamond structure boundaries behind to seek their fortune in the hippie camp that is called liquid.

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It's true that the high melting temperature of diamond is caused by its structure. Usually a substance melts when you overcome the intermolecular attraction forces, whereas for diamond the forces involved are interatomic, which are several orders of magnitude stronger. However, one could argue that fusion is a physical process, and in this case we're breaking chemical bonds so it's more of a chemical process which would make it, technically, not fusion.

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