Polyatomic ions and their charge

[icefire23]

This question is regarding polyatomic ions and their charge. I have always been told that I have to memorize the charge of polyatomic ions. The question is whether I can determine by myself the charge of two atoms when they join together to form a polyatomic ion. For example Se and O can form selenate SeO4, this polyatomic ion has a charge of -2. I would like to know why is the charge -2? But what I know the sum of all oxidation states of the joining ions must be equal to the net charge of the ion. But what happens then if I do not know the net charge of the ion? Hopefully these questions are not too silly. Thanks before hand, Aldo.

 

[Bloodthunder]

Because the ion is like this. There is 1 unpaired electrons on 2 oxygen atoms, giving it a charge of 2-.

For these kinds of ions, just try pairing up their electrons to get a stable electron configuration until you only have unpaired electrons left. You can then deduce the charge of the whole ion from that.

 

[icefire23]

Ok, seeing the bonds in the ion helped. Thank you very much for your help. So, when I have the SeO3 anion with a charge of -2, I would have 1 Selenium Oxygen double bond and two Selenium Oxygen single bonds, with an electron pair in the Selenium? Aldo.

 

[Bloodthunder]

yeap.

btw, what's aldo?

 

[icefire23]

Thanks again for your help. Hahahaha..Aldo is just my name : )

 

[Natko]

Because the ion is like this. There is 1 unpaired electrons on 2 oxygen atoms, giving it a charge of 2-.

For these kinds of ions, just try pairing up their electrons to get a stable electron configuration until you only have unpaired electrons left. You can then deduce the charge of the whole ion from that.

I still don't get how SeO4 acquires a charge of 2-. I don't understand the diagram above either. Please explain in more plain language.