Understanding Polyatomic Ion Charges and Number of Oxygen

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I have recently started self studying chemistry and I am at the point where I'm learning to name chemicals given chemical formulas and vice versa. (Like Iron (III) Oxide --> Fe2O3). I am having trouble with doing this process for polyatomic ionic compounds. I can't seem to remember some the charges and the number of oxygen associated with the PA ion. I learned about PA ion naming just today and I could probably memorize the charges and number of oxygens pretty easily but that doesn't give me satisfaction. Is there any method to derive these charges and number of oxygens?

Ex - going from Sulfate to SO4 2- without memorizing the 4 or 2- charge, just knowing Sulfur and "-ate"
 

Borek

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Systematic naming of polyatomics exists, but virtually nobody uses it in the real life. For historical reasons common names are full of inaccuracies and ad hoc rules, as they evolved with our increasing knowledge. They are far from being systematic and logical and it is better to memorize them.
 
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Not easy.
General rule is:
"-ate" is the highest oxidation state if less than +7
There are additional specifications of "ortho" and "meta" if there are several ions of the same oxidation state. But these are commonly omitted if one species is common.
 

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