Polyatomic ion charge?

Boston

How do I work out the charge on a polyatomic ion. Like why does a sulfate and sulfite have the same charge. I keep going through methods that might be why, but then it doesn't work for other Polyatoic ions.

O and is there anyway of knowing how many oxygen atoms are in a oxoanion, I know ate means more ite means less, per means most and hypo means least, but is there anyway of knowing that sulfate has four oxygen atoms.

Probably simple questions but I'm not that sharp when it comes to chemistry.

planck2

Its been a while since I've done chemistry , but there should be a set of valence rules in the chemistry book. Sorry I can't be of more help. If you need any more chemistry help send Dataisgod a private message he's a chemistry whizz, but he's on holidayat the moment so I dont know what his email access is like

Capt'n Midnight

Part of the reason is that Sulphur can have different valencies so can bind to different numbers of oxygen atoms. - once you get to elements with a third main shell then the outer electrons are somewhat shielded from the nucleus so are more free to do play tricks on unsuspecting students.

I don't think there are any hard and fast rules - the names are the way you remember them, most of the compounds are acids so if you remember the formulae of the acid it should help - H2SO4 (2)

see also
ferric / mercuric / cupric
ferrous / mercurous / cuprous


one way of looking at it that Ozone has three oxygen molocules each sharing ONE electron with the other two - so the oxygen atom does not always share two electrons with another atom (inaccurate analogy of course)

planck2

That is a load of bull.

Boston

I'm rather confused, Chemistry aint my thing, and this is bugging me.

Zaphod

Boston wrote:

How do I work out the charge on a polyatomic ion. Like why does a sulfate and sulfite have the same charge. I keep going through methods that might be why, but then it doesn't work for other Polyatoic ions.

Sulfur (and other elements) can have variable valencies.The -ate and the -ite refer to the relative oxidation state, -ate being higher, -ite being lower.

So sulfate anion is SO4 (2-) [S has valence of +6] and sulfite anion is SO3(2-) [S has valence +4] - oxygen has valence -2 throughout as oxygen is more electronegative than sulfur (i.e. oxygen has a stronger hold on its electrons than sulfur). Hence sulfuric acid is H2SO4, and sulfurous acid is H2SO3 (-ic and -ous are synonymous with -ate and -ite). The variable valence has got to do with relative stability/energy levels which are dependent on the way the orbitals are filled out e.g empty orbitals vs. full orbitals vs. half-filled etc.

S(+0) has 16 electrons so we get 1s2, 2s2, 2p6, 3s2, 3px2, 3py1, 3pz1 i.e. a partially filled 3p orbital is not 'happy'. e.g. elemental sulfur

S(+4) has lost 4 electrons giving 1s2, 2s2, 2p6, 3s2. The 3p orbital is empty and 'happy' and all the other orbitals are full (1s, 2s, 2p, 3s) i.e. more stable (energetically favoured) configuration. e.g. sulfite

S(+6) has lost 6 electrons giving 1s2, 2s2, 2p6. The 3s2 is empty and all the other orbitals are full (1s, 2s, 2p) -> stable configuration. e.g. sulfate


You can see from the above that we can also achieve a stable configuration by adding electrons.

S(-2) has gained 2 electrons giving 1s2, 2s2, 2p6, 3s2, 3px2, 3py2, 3pz2. Now the 3p orbital is full -> stable configuration. e.g. sulfide

Boston wrote:

O and is there anyway of knowing how many oxygen atoms are in a oxoanion, I know ate means more ite means less, per means most and hypo means least, but is there anyway of knowing that sulfate has four oxygen atoms.

Probably simple questions but I'm not that sharp when it comes to chemistry.

No, there's no way of 'knowing' - you just have to learn them off or else learn off the valencies of the elements and work from that (i.e. as +6 is the highest oxidation state of sulfur, then -ate will refer to that). Fadó, fadó, when I did my first class in Leaving Cert chemistry, the first nights homework we got was to learn off the names, formulae and charges of the most common polyatomic ions eg sulfate, sulfite, nitrate, nitrite, phosphate, carbonate etc.

GuanYin

planck2 wrote:

That is a load of bull.

While you are probably right, can you include some information to show why next time?

planck2

syke wrote:

While you are probably right, can you include some information to show why next time?

I'll try, but since physics and mathematics are more my thing, I'd rather leave any technical chemistry details to my friend data since although i can point out wrong general answers I won't relly know the technically correct answer.

Boston

Thank you Zaphod