Determining Charges of Polyatomic ions

[Tclack]

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)

 

[Borek]

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^-, SO₃^2-, SO₄^2-, PO₄^3-, CO₃^2-, perhaps BO₃^3-. 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.

 

[Tclack]

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.

 

[Borek]

Don't blame just transition metals, throw in periodic acid with two different forms depending on pH: IO₄^- and IO₆^5-