|MadSci Network: Earth Sciences|
Your question is one that exhibits the failure of language to be as precise as science can be. That is, what do you mean by "strong?" Hardness? Diamonds are very hard, but they can be shattered. Compressive strength? Concrete takes enormous compressive loads but fails fairly easily with shear or tensile stress. And, are we talking strength for a given diameter of material, or a given mass of it (makes a big difference comparing metals to less dense materials).
And in what conditions are we making the measurement? (Using diamond again as an example, in a hot oxidizing atmosphere, diamond will burn, where "lesser" materials like aluminum oxide will survive. So in that sense, in those conditions, zirconium oxide is 'stronger' than diamond.).The physical form can also have a bearing on the issue. The polycrystalline form of a given material is often less prone to fail under stress than the same material in a single crystalline state; that's because cracks in one little crystal grain don't propagate as easily across grain boundaries as they do in a single crystal. Both are the same material, same atoms bonded in the same way, but they behave differently under mechanical stress.
I'm not trying to confuse you (though I may have, unfortunately, done that) but alert you to the fact that "strong" is not a measureable physical property.
Now, all that said, it is a general truism that the stronger (in electron volts of energy) the chemical bonds in a material, and the shorter (in nanometers or picometers) those bonds are, the stronger the material holds together. In this regard, diamond (despite its brittleness arising from its crystalline nature) is a tremendously strong material. It is only exceeded by its cousin graphite, and then only is a very specialized sense. In a single plane of carbon atoms formed into graphite, the bond lengths are even shorter than in diamond, and thus (in that direction) graphite is the chemically strongest of all materials. Of course, these single planes of graphite are only very weakly bonded with each other and slip readily, so that in a mechanical sense, graphite is a very weak material and slides so readily it can be used as a lubricant. There's a real paradox for you: a material that is at the same time both the chemically strongest and one of the mechanically weakest!
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