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I came across a compound $\ce{[FeO4]^{2-}}$ but $\ce{Fe}$ has +6 charge according to my calculations. How this is possible ? Or it is possible but my periodic table is kind of non-detailed one. (On the table I got $d$-metals possible charges).

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If you write out the electron configuration of Fe, you will find it has $3\text{d}^6 4\text{s}^2$ in its valence level, meaning it can theoretically take on any oxidation state from +1 to +8 (and even some negative states, for that matter).

In practice it is extremely uncommon to find any oxidation states other than +2 or +3 for iron - which explains why a simple periodic table containing only the most common naturally occurring states will omit them - but they are possible, just not very stable or long-lived. The polyatomic ion you have observed is called ferrate.

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  • $\begingroup$ I've always enjoyed citing catalase as an example of a ubiquitous compound where iron takes on uncommon oxidation values. $\endgroup$ – Nicolau Saker Neto Jun 1 '13 at 20:40
  • $\begingroup$ @NicolauSakerNeto Very interesting example! $\endgroup$ – kaliaden Jun 3 '13 at 3:58

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