There is a chemical reaction describing the oxidation of $\ce{Fe3O4}$ (magnetite) to $\ce{Fe2O3}$ (hematite). It is: $$\ce{2Fe3O4 (s) + 1/2 O2 (g) ->[\Delta] 3 Fe2O3 (s)}$$ My question is what does $\ce{1/2 O2}$ mean? Is it like a single oxygen atom or what and if it is why not just typing $\ce{O}$ instead of $\ce{1/2 O2}$?


Although by definition, it means that for every two $\ce{Fe3O4}$ molecules you need half an oxygen molecule, this does not translate to the real world, as you either have one or more molecules, or none at all.

With equations like these, to know how many of each molecule are actually needed for the reaction to proceed, you have to make sure all of the coefficients are whole numbers. In this case you can do this by multiplying all of the molecule coefficients by 2. Which will give you this:

$$\ce{4Fe3O4 (s) + O2 (g) ->T[heat] 6Fe2O3 (s)}$$

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    $\begingroup$ Fe3O4 is in no way a molecule. A correct way of saying about this is referring to the moles. $\endgroup$ – wolphram Oct 24 '17 at 8:13
  • $\begingroup$ @wolphram While I agree with your sentiment of the criticising that $\ce{Fe3O4}$ is not a molecule, referring to the amount of substance as moles is at least equally as wrong. $\endgroup$ – Martin - マーチン Oct 24 '17 at 8:18
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    $\begingroup$ @Martin-マーチン I don't see how "a mole of formula units" can be more wrong $\endgroup$ – wolphram Oct 24 '17 at 8:19
  • $\begingroup$ @wolphram You did not write that. The coefficients in a chemical equation are proportional to the amount of substance which is measured in mole. If you would like to discuss this further, I kindly invite you to Chemistry Chat. $\endgroup$ – Martin - マーチン Oct 24 '17 at 8:22
  • $\begingroup$ @wolphram considering the question OP asked, I wasn't sure if they knew about moles. I agree that Fe3O4 is not a molecule, but I'm unable to think of a better word (especially one that is simple/understandable) $\endgroup$ – epelito Oct 24 '17 at 8:33

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