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Determining Charges of Polyatomic ions

  1. Jan 23, 2014 #1
    Does there exist a fool-proof way of doing it?

    I've scoured the forums, and I've already found this question, but the answers are not satisfying.

    Here for example is exactly my question:

    I've already memorized a bunch of -ate's (nick the camel... and i've made my own to cover more)

    My assumption is that you look at each atom's ion state and add them up. This works for some, but not others. Nitrate for example (NO3-).
    A Nitrogen ion is -3 (or +5)
    And each Oxygen is -2 (or +6)
    So I can see +5 and -6 makes: -1, Which works out!

    It doesn't work for Nitrite however (NO2-)
    +5 plus -4 leaves a +1

    So, Why is what I originally thought wrong?
    and how does one determine the charges of polyatomic ions? (specifically oxyanions... for now)
  2. jcsd
  3. Jan 24, 2014 #2


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    Staff: Mentor

    There is a good new, and a bad news.

    Bad news is - there is no universal method.

    Good news is: it doesn't matter (much).

    As you have already found, final result depends on the oxidation state on the central atom, and we can determine it knowing the polyatomic charge - so using it the other way around won't work.

    However, in reality there are just a few polyatomic ions that are commonly used, so it is easier to remember them, than to look for elaborate schemes. NO3-, NO2-, SO32-, SO42-, PO43-, CO32-, perhaps BO33-. All those containing chlorine or bromine will have a charge of -1. There are some tricky phosphoric acids, but they are rarely used and I always just remembered they exist and I have to check the details once I need them.
  4. Jan 24, 2014 #3

    It's on wikipedia and it's not sourced, but...


    Does this make any sense? Sulfate is SO4^2−
    But, let's assume we don't know that charge, and someone told us to write sulfate. So, it's not in group 7, so the central atom of S will have a 6+ charge. With 4 oxygens at -2 giving a -8 ox. number, the total is -2!

    Superficially, this makes sense (I haven't exhaustedly checked every one), but if it is true, am I correct in assuming that all* oxyanions of the same central atom have the same charge?

    So far I've found:
    nitrate and nitrite have a -1 charge
    phosphate and phosphite have a -3
    perchlorate, chlorate, chlorite, hypochlorite all have -1 charge
    Sulfate, sulfite, hyposulfite have a -2 charge
    Arsenate, arsenite have a -3 charge
    perbromate, bromate, bromite, hypobromite all have a -1 charge
    Perselenate, Selenate and selenite have -2

    *I assume that transition metals do their own thing, because I've found
    Permanganate (MnO4^1-) and Manganate(MnO4^2-) apparently, Permanganate is formed from the Mn7+ ion and Manganate from the Mn6+ ion. Stupid transition metals ruin predictive properties.*

    One thing I'm trying to resolve is the determination of the -ate suffix.
    There's two possible origins:
    1. The naming rule from wikipedia. (Halogens get per_ate, the others get _ate if oxy #'s = grp #'s)
    2. What I've always heard, that the most common oxyanion gets the -ate suffix

    It seems unlikely that these are both correct. I've read #2 in at least two textbooks and I've only found #1 on wikipedia. This makes #2 more likely. However, checking a bunch of them gives merit to #1. Does anyone know of another source? I'd like to cite it if possible.
    Last edited by a moderator: Jan 24, 2014
  5. Jan 24, 2014 #4


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    Staff: Mentor

    Don't blame just transition metals, throw in periodic acid with two different forms depending on pH: IO4- and IO65-
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