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Having a real pain with nomenclature. Look at this substance:

$$\ce{MnO4-}$$

So, do we have an anion of manganese and oxygen, or is that just an anion of oxygen? Anyway, my book says that $\ce{MnO4-}$ is "permanganate".

Since it ends with "-ate", it seems my book thinks $\ce{MnO4-}$ is an oxyanion (because those are the ones that end with things like "-ate" and "-ite").

However, it was my understanding that an oxyanion is when you have oxygen and a non-metal. But manganese is a metal! What is going on here then?

And not just that. Let's assume that $\ce{MnO4-}$ is indeed an oxyanion called permanganate. From what I've learned, if you remove one oxygen atom (getting $\ce{MnO3-}$), it should become just manganate, right? But, according to my book, manganate is $\ce{MnO2-}$. It had to lose two oxygen atoms and not one to become manganate. Why?

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  • $\begingroup$ Afaik, manganate is actually $\ce{MnO4^2-}$ $\endgroup$ – Jan Sep 21 '17 at 6:39
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As far as I know, an oxoanion just refers to an anion which has oxygen covalently bonded to the central atom; the central atom can be pretty much anything apart from the Group 1 and 2 metals, since the only oxygen compounds those form are ionic oxides/peroxides/superoxides. For the 3d transition metals, you have $\ce{VO3^-}$ (vanadate(V)), $\ce{CrO4^{2-}}$ (chromate(VI)), $\ce{MnO4-}$ (manganate(VII)), and $\ce{FeO4^{2-}}$ (ferrate(VI)), amongst others.

I would not recommend reading too much into the nomenclature, because a lot of it is just what chemists are comfortable with after using for a long time - it's like a language that you just get more familiar with over time. But there are some general rules. The 'per...ate' name, such as perchlorate $\ce{ClO4-}$, implies a higher oxidation state than the normal '...ate' compound, which is chlorate $\ce{ClO3-}$ in this case. For chlorine, only certain oxidation states are available, namely the -1, 0, +1, +3, +5 and +7 oxidation states. Apart from 0, which occurs in diatomic chlorine $\ce{Cl2}$, you will notice that these are in increments of 2 - which means that you can generate the whole list of chlorine anions by adding one more oxygen, but keeping the charge of the anion constant: $\ce{Cl-}$, $\ce{ClO-}$, $\ce{ClO2-}$, $\ce{ClO3-}$, $\ce{ClO4-}$. This tendency to form compounds 2 oxidation states apart is very pronounced for the p-block elements, i.e. Group 15, 16 and 17.

However, for d-block elements such as $\ce{Mn}$, this isn't the case because single d electrons are easily lost and gained. So after permanganate, $\ce{MnO4-}$, which features $\ce{Mn}$ in the +7 oxidation state, the oxoanion with the next highest oxidation state is the manganate ion $\ce{MnO4^2-}$ which has $\ce{Mn}$ in the +6 oxidation state. (As far as I know the ion $\ce{MnO2-}$ is not called "manganate".)

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An oxyanion is not necessarily a non-metal and oxygen, rather, it is more simply a chemical element and an oxygen. There are a large number of elements that form oxyanions, many of which are listed here.

To quote this Chem.SE post regarding a the difference between manganate and manganite:

Manganese oxidation state nomenclature is a real world mess. Drawing nomenclature parallels to $\ce{Cl(VII)}$ and downward is not valid. Drawing nomenclature parallels to titanium is poor, for $\ce{Ti(IV)}$ is the highest common oxidation state. $\ce{SrO +TiO2}$ gives $\ce{SrTiO3}$, properly called titanate, for it is $\ce{Ti(IV)}$ on both sides.

For someone who is new to chemistry, the only metal oxyanions I would recommend that you learn are chromate ($\ce{CrO4^2-}$), dichromate ($\ce{Cr2O7^2-}$), and permanganate ($\ce{MnO4^-}$).

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  • $\begingroup$ What a huge mess! In other words, I ought to be learning most compositions out of memory D: $\endgroup$ – Voldemort Jun 16 '15 at 23:35
  • $\begingroup$ So an oxyanion is basically oxygen with whatever as long as it is an anion? (The oxygen an anion, or the other thing an anion?) $\endgroup$ – Voldemort Jun 16 '15 at 23:36
  • $\begingroup$ That is correct. Luckily the non-metal oxyanions behave nearly all the same. The metal oxyanions are the exception. $\endgroup$ – ringo Jun 16 '15 at 23:38

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