How do you know the charge of polyatomic ions?

Working out the charge on a polyatomic ion.?

• 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?

• Answer:

  The real way of finding anything in chemistry has to be by experiment. You are asking about predicting the probable charge which is different. Most positive polyatomic ions are made by adding H+ to a molecule with a lone pair. Adding 2 H+s is unlikely. For negative ions make as many bonds as possible with the neutral atoms then add electrons to fill in the gaps. The number of electrons is the negative charge. Eg PO4 Phosphorus can make up to 5 bonds with its 5 outer electrons. A double bond to one oxygen and single bonds to the other 3. Those 3 oxygens are left with 7 outer electrons so can each gain another: PO4 3- Another useful idea is "isoelectronic" - the same number of electrons. N2 is neutral. CN would have one fewer electron so adds one to make CN- CC adds two to form the Carbide ion C2 2- in for example CaC2 CO is isoelectronic with N2 so does not need to gain or lose electrons.