Determining Charges of Polyatomic ions

  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:
    https://www.physicsforums.com/showthread.php?t=525291&highlight=polyatomic+ions

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

    HERE'S WHAT I ORIGINALLY THOUGHT:
    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. Borek

    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. I FOUND SOMETHING!

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

    http://en.wikipedia.org/wiki/Oxyanion#Naming

    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. Borek

    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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