# Determining Charges of Polyatomic ions

by Tclack
Tags: charges, determining, ions, oxyanions, polyatomic, polyatomic ions
 P: 37 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: http://www.physicsforums.com/showthr...olyatomic+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)
 Admin P: 22,712 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.
 P: 37 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.