Take, for istance, phosphorus; it has eight different oxidation numbers, but it can only form two different anhydrides, phosphoric (when the oxidation number is 5) and phosphorous (when it is 3). How do I understand which ones I can use to get anhydrides? Or take chlorine, it also has eight oxidation numbers but if I wanted to know the corresponding hydracid, HCl, how'd I know which number to use?

Please forgive me if the question is trivial and written in a poor english, but I'm having trouble with both chemistry and English language classes at present!

  • $\begingroup$ Truth be told, in general case you cannot know this beforehand. Some general rules do exist, but everyday chemistry mostly works with special cases, that do not follow rules very well. And oxidation number is a derived characteristic anyway. $\endgroup$
    – permeakra
    Jan 14, 2015 at 13:09
  • $\begingroup$ Inorganic anhydrides are rather simply called oxides and during hydrolysis usually oxidation numbers don't change (however in case of ClO2 and ClO3 disproportionation occurs). $\endgroup$
    – Mithoron
    Jan 14, 2015 at 13:26

1 Answer 1


You need to be familiar with some of the compounds earlier, otherwise you can't tell if it exists. Remember facts come first before explanation. It also depends usually on the atom/ion to which it is bonded. for example since Fluorine is most electronegative it can form $\ce{XeF6,XeF4}$,etc. but $\ce{XeH3}$ doesn't form also oxygen forms large oxidation states as it is electronegative too and there is also more energy released as many bonds are formed (ususlly pi-bonds.)


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