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For some reason, stuff made entirely of carbon atoms tends to be incredibly strong, beyond just about anything else known to material science. It doesn't seem to matter too much how the carbon is arranged either; whether it's a tetrahedral crystal lattice (diamond), a hexagonal 2D lattice (graphene), or a soccer ball-shaped spheroid (buckyball), pure carbon seems to magically be super-strong in just about any configuration, except graphite.

So what it it about carbon that makes it super-strong, and what it it about graphite that screws up this property?

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Carbon is not universally super-strong. True, diamond sets a record in hardness, but: (1) it is not that far beyond anything else, as some materials (cubic boron nitride, etc.) come pretty close, and (2) hardness is not the same as strength. Also, there are different kinds of strength.

Graphene is a 2D material, which is an entirely different realm, so it can't really be compared to the rest.

Buckminsterfullerene is neither hard nor strong at all; you may easily crush a piece of it on your fingernail. Some of its derivatives display certain promising characteristics on a microscale, but there is a long way between that and a bulk material.

All that being said, carbon has some properties that make it special: it tends to form covalent bonds, and it prefers to have four of them. Not that the carbon-carbon bonds are record strong (none of them is as strong as the triple bond in $\ce{N2}$), but this certainly paves a way to a lot of interesting combinations, as illustrated by many allotropes of carbon (probably more than in any other element) and the immense diversity of organic chemistry.

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  • $\begingroup$ The (en.wikipedia.org/wiki/Boron_nitride) boronitride you mentioned is very interesting in comparison to diamond. Isoelectronic, same lattice motif, comparable density. And it can even form nanotubes, similar to the carbon. $\endgroup$ – ssavec Sep 30 '15 at 6:02
  • $\begingroup$ Sure. One may almost say it is a "carbon in disguise". It even has organic chemistry too. $\endgroup$ – Ivan Neretin Sep 30 '15 at 6:06
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Part of the strength of C-C bonds, especially bonds in cyclic carbon molecules, is from the sigma bond. This type of bonding has a high degree of symmetry, as in the Kekule benzene diagram. It also blends electrons in multiple orbitals as a single hybrid orbital.

Although the structure can be drawn in alternative symmetrical diagrams, e.g. mirror images of each other, according to quantum mechanics the superposition of both states occur simultaneously (unless one forces it to resolve into a single state, such as by a chemical reaction or by measurement). This distributed nature adds to the strength of the bond.

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  • $\begingroup$ Kekule benzene thing is about pi bonds, not sigma. $\endgroup$ – Ivan Neretin Sep 30 '15 at 16:49

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