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I am just beginning in chemistry and learning how to name the different molecules. I have this molecule: $\ce{PbCrO4}$.

I would give it the following name: lead(II) chromate(VI) tetraoxide. Lead and chrome have both multiple oxidation states. And as you have to specify about which ion you are speaking, it seems to me this would be the most correct name.

But this is the correct name apparently: lead(II) chromate.

Could someone please explain:

  • why you just specify the oxidation state for the first element and not the other? As they are both metals and both have multiple oxidation states.

  • why you don't say tetraoxide, although there is clearly a number 4 written?

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Chromate ($\ce{CrO4^2-}$)is a poly atomic ion which is partially defined by the oxidation state of chromium. Its like the sulfate ($\ce{SO4^2-}$), phosphate ($\ce{PO4^3-}$), nitrate ($\ce{NO3-}$), permanganate($\ce{MnO4-}$), and carbonate ($\ce{CO3-}$) polyatomic ions. For each ion the oxidation state of the center ion is defined therefore there is no reason to state it. You could in principle call lead (ii) chromate lead(ii) chromium(vi) oxide or tetroxide, but this is less informative of the structure of the dissolved species.

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  • $\begingroup$ So you actually never say the oxydation state of the ions in the middle, just the first one? This might be an ever more stupid question but, how do you know it's $CrO^{2-}_{4}$ and not $Pb^{2+}Cr^{6+}O^{2-}_{4}$? $\endgroup$ – privetDruzia Jan 9 '16 at 15:43
  • $\begingroup$ No, not the middle, the $\ce{CrO4^2-}$ is a stand alone polyatomic ion, which will not hydrolyze in solution. This is different from a mixed oxide like yttrium aluminium oxide ($\ce{Y3Al5O12}$) which is two oxides mixed together like you're thinking. A good way to distinguish between mixed oxides and salts is if the stoichiometry can vary or if it is fixed respectively. $\endgroup$ – A.K. Jan 9 '16 at 16:20
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The simple answer is that there is naming convention for polyatomic anions.

  • $\ce{S^{2-}}$ is sulfide
  • $\ce{SO3^{2-}}$ is sulfite
  • $\ce{SO4^{2-}}$ is sulfate
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