When writing the formula for an ion, should the operator come before or after the number that indicates the ion's charge?

Here are some examples:

$\ce{SO_4^{-2}}$ or $\ce{SO_4^{2-}}$ $\ce{Fe^{+2}}$ or $\ce{Fe^{2+}}$

I'm not great at chemistry, so I'm sorry if it's a stupid question.

I just haven't been able to find many answers.

  • 2
    $\begingroup$ $\ce{SO4^{2-}}$ and $\ce{Fe^{2+}}$ $\endgroup$
    – MaxW
    Jun 9, 2018 at 13:44
  • $\begingroup$ @MaxW I have made your absolutely correct comment into an answer. I hope you do not mind. Just trying to return friendly answers to new users. Can delete it if you feel uncomfortable $\endgroup$ Jun 9, 2018 at 15:07

1 Answer 1


As @MaxW has said in comments, the correct form is:

$\ce{SO_4^{2-}}$ and $\ce{Fe^{2+}}$

It is just as in math. If you have $a+a$ it results in $2a$, not $a2$, and thus:

$\ce{SO_4^{--}=SO_4^{2-}}$ and $\ce{Fe^{++}=Fe^{2+}}$

It is not a stupid question. IMHO it is a perfectly legitimate question. I apologize for those who are downvoting your question.

  • $\begingroup$ Last I checked it was -1 + -1 = -2 not 1- + 1- = 2-, so "just as in math" is a slightly flimsy argument. $\endgroup$
    – R.M.
    Jun 9, 2018 at 15:39
  • $\begingroup$ @R.M. $a+a=2a$. Charges are entities, not numbers. $\endgroup$ Jun 9, 2018 at 15:42
  • 1
    $\begingroup$ My point was that "just as in math" is somewhat dismissive, as you can come to the exact opposite conclusion from math conventions from the -1 + -1 = -2 argument (which is probably where most people who want to put the sign in front of the number get it from). -- "Charges are entities, not numbers" might be the rationale, but currently it appears nowhere in your answer. $\endgroup$
    – R.M.
    Jun 9, 2018 at 15:46

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