[disillusioned19]

How does one go about finding the charge on a polyatomic ion? For example we have CN- but then we have a CO neutral molecule. Both species follow the octet rule, but the cyanide molecule has a negative charge whilst the carbon monoxide compound is neutral. Is it due to the way that they're made from constituent ions as opposed to atoms?

Many sites and books deal with the reverse thinking of what I've proposed i.e. they give an example where the formula and charge are already known, and so the structure is worked out from there. I'm suggesting how does one know the charge beforehand?

[Mobius1988]

Hmmm well you can generally do it by knowing how many bonds the atoms will have between each other and then placing positive and negative charges where required to make it up to 8 electrons. The best way to describe it is probably with examples.

So take N_3^- as an example. Pretending that you dont know there is a negative charge. First add single bonds between the 3 nitrogens:

N-N-N

You know that each nitrogen atom can have a maximum of three bonds, but they can also have 4 bonds and be a postiviely charged quaternary nitrogen. So add another bond to each nitrogen:

N=N+=N

At this point you cant add any more bonds because nitrogens cant have 5 bonds. The nitrogen on each end has 2 electrons in the bonds, is sharing 2 electrons from the central nitrogen and has 3 electrons left giving a total of 7 electrons. So now the only way to make them obey the octet rule is to add an electron to each nitrogen bringing the total number up to 8 electrons and putting a negative charge on each nitrogen.

^-N=N⁺=N^-

Count up the charges and you have N₃^-. Its all about counting the electrons while making sure you dont break bonding rules.

Hope that made sense.

[Mobius1988]

If you are at uni they may teach you VSEPR Theory which will help you understand this (Valence Shell Electron Pair Repulsion Theory).

[grs35]

may be its not the answer you want to hear, practicing makes it easier. do as many as you can. they dont have to be in the problems section in your book. just grab a bottle of a juice and try to write down the compounds you see. it really helps.

[disillusioned19]

Hmmm well you can generally do it by knowing how many bonds the atoms will have between each other and then placing positive and negative charges where required to make it up to 8 electrons. The best way to describe it is probably with examples.

So take N_3^- as an example. Pretending that you dont know there is a negative charge. First add single bonds between the 3 nitrogens:

N-N-N

You know that each nitrogen atom can have a maximum of three bonds, but they can also have 4 bonds and be a postiviely charged quaternary nitrogen. So add another bond to each nitrogen:


N=N+=N

At this point you cant add any more bonds because nitrogens cant have 5 bonds. The nitrogen on each end has 2 electrons in the bonds, is sharing 2 electrons from the central nitrogen and has 3 electrons left giving a total of 7 electrons. So now the only way to make them obey the octet rule is to add an electron to each nitrogen bringing the total number up to 8 electrons and putting a negative charge on each nitrogen.

^-N=N⁺=N^-

Count up the charges and you have N₃^-. Its all about counting the electrons while making sure you dont break bonding rules.

Hope that made sense.

Yes thankyou that makes a lot of sense! Sorry for the belated thanks - haven't been as active on here in recent weeks.

Thanks again!