Would 2 C₂ be 2 moles of diatomic carbon, or 4 moles of carbon?

Would molecules like $\ce{C2, H2}$, etc., be considered 1 mole of diatomic [element] or 2 moles of [element]?




You can have 1 mole of $\ce{C\_2}$ which contains 2 mole of $\ce{C}$ just as you can have for example 1 mole of $\ce{C\_2H\_5OH}$ which contains 2 mole of $\ce{C}$, 6 mole of $\ce{H}$ and 1 mole of $\ce{O}$.


The point is that you always have to mention whether you are indicating moles of molecule or moles of atoms. That is why a statement like: "I have 1 mole of oxygen" is ambiguous, because it could be referring to oxygen gas (i.e. $\ce{O\_2}$) or to atomic oxygen (i.e. $\ce{O}$).


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$\ce{C\_2}$ is the blue-green Swan band emission of comets' ion tails and the blue in gas flames. The big question is its electronic structure (presumably not $\ce{•C#C•}$). Its paper synthesis in cryogenic gas matrix (UV cleavage) or in vacuum (flash pyrolysis or UV) seems to be easy, from


ROO-C(=O)-C#C-C(=O)-OOR, the peresters of acetylenedicarboxylic acid with R = $\ce{-CF\_3}$, $\ce{-C(CH\_3)\_3}$, or $\ce{-Si(CH\_3)\_3}$.


Strings of alpha-omega diradical polycarbyne, $\ce{(-C#C-)\_n}$ or $\ce{(=C=C=)\_n}$ (your choice in LCAO, same in MO), in carbon arcs are suggested to pericyclicly close to fullerenes. 


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