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Let's say I want to find the oxidation numbers of the atoms in a chemical formula.

For example: In $\ce{H3AsO3}$,

Oxygen has a $-2$ charge. They're $3$ of them, so its $-6$

Hydrogen has a $+1$ charge. They're $3$ of them, so its $+3$.

So arsenic must have a charge of $+3$ for all the charges to add up to be $0$? Correct?

However, my professor keeps telling me to keep the positive oxidation numbers away from each other. I thought it only applied when drawing Lewis structures or resonance structures.

Is that a rule when writing a formula?

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    $\begingroup$ He could have told that two vowels can't be next to each other in a word. That's about as reasonable. $\endgroup$
    – Ivan Neretin
    Commented Mar 27, 2020 at 8:23

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Can two positive ions be next to each other in a formula?

Yes, there are many examples of this, $\ce{HNO3}$ for instance

So Arsenic must have a charge of 3+ for all the charges to add up to be 0? Correct?

Yes, that's correct

However, my professor keeps telling me to keep the positive oxidation numbers away from each other. Is that a rule when writing a formula?

It's not a rule that I'm familiar with. Here is a link to a nice, concise listing of the rules for writing formulas for inorganic compounds.

I thought it only applied when drawing Lewis structures or resonance structures.

Yes, it certainly applies in those two cases

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  • $\begingroup$ I think in $\ce{HNO3}\equiv\ce{(HO-)N(=O)2}$ no two positive partial charges are next to each other, so that would not provide an example, but $\ce{HNO}$ would. $\endgroup$
    – Martin - マーチン ♦
    Commented Jul 17, 2014 at 2:55
  • $\begingroup$ So would you say that I can write $\ce{H3AsO3}$ as $\ce{AsH3O3}$ and it wouldn't make a difference because my Lewis diagram would remain the same? $\endgroup$
    – Mark
    Commented Jul 17, 2014 at 2:58
  • $\begingroup$ @Martin I meant it in the nomenclature sense. In other words, in the molecular formula an $\ce{H}$ (+1) is written next to an $\ce{N}$ (+5) $\endgroup$
    – ron
    Commented Jul 17, 2014 at 2:58
  • $\begingroup$ @Mark No, see the rules for constructing molecular formulas in the link I provided. $\endgroup$
    – ron
    Commented Jul 17, 2014 at 3:00
  • $\begingroup$ @Mark There are the official rules for writing sum formulas, like in the link that ron provided. There are some other ways of writing them, like the Hill system that is quite common in Organic Chemistry. Also the Lewis formula is only one representation of a molecule, and there are a lot of examples where this approach fails (see hypercoordinate species).// ron - I guess I did not see that it is a question about the notation, I thought this would be a question about actual charges. $\endgroup$
    – Martin - マーチン ♦
    Commented Jul 17, 2014 at 3:10

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