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Why does hypophosphoric acid has a strange name? My knowledge of inorganic nomenclature tells me that the prefix "per-" should be used with "-ic" acids and the prefix "hypo-" goes with "-ous" acids, which isn't the case here. Why is this so?

Just as part of an additional observation, two other things which I find strange about $\ce{H4P2O6}$ are:

  • I think this is the only oxy-acid of phosphorus having a $\ce{P-P}$ bond.
  • It has phosphorus in $+4$ oxidation state.
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    $\begingroup$ Don't look for logic in chemical naming, it is not always there. Maybe it's just because all shorter names were taken. $\endgroup$ – Ivan Neretin Apr 20 '17 at 10:33
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    $\begingroup$ @IvanNeretin Thanks, I'm always at a hunt for logic, I think I must curb this habit of mine atleast in nomenclature $\endgroup$ – Berry Holmes Apr 20 '17 at 10:45
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    $\begingroup$ Chemists use whatever fits. Nitric acid = oxidation state of nitrogen is +5, nitric oxide = +2, nitrous acid = +3, nitrous oxide = +1. Perchlirate and permanganate each = +7 for the root element, but chlorate = +5 and manganate = +6. And so on. $\endgroup$ – Oscar Lanzi Apr 20 '17 at 10:48
  • $\begingroup$ chemistry.stackexchange.com/questions/40506/… $\endgroup$ – Nilay Ghosh Apr 20 '17 at 11:09
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    $\begingroup$ @IvanNeretin That is bad advice. There is pretty much always some logic behind the names, even if we know today it's wrong. And it always helps to know what the error was, if only to not fall into the same trap. $\endgroup$ – Karl Sep 24 '17 at 14:38
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You are most of the way to the reason yourself:

$$\begin{array}{|c|c|c|}\hline \text{name} & \text{formula} & \text{oxidation number of P}\\ \hline \text{phosphoric} & \ce{H3PO4} & +5\\ \text{phosphorous} & \ce{H3PO3} & +3\\ \text{hypophosphorous} & \ce{H3PO2} & +1\\ \hline \end{array}$$

The "hypo-" prefix means "beneath" or "less than". Hypophosphoric acid, $\ce{H4P2O6}$, has oxidation number of phosphorous as +4, which is in between phosphoric acid and phosphorous acid. Thus, "hypophosphoric" refers to "beneath" phosphoric (but above phosphorous) acid.

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