Question

When you're drawing a lewis structure for a polyatomic ion, how do you know which atom gets the extra negative/positive charges? Thanks!

Answer 1

Generally the more electronegative atoms get the negative charge since this is the most stable resonance structure (conversely, less electronegative elements tend to gain a positive charge). For instance, many polyatomic ions contain oxygen, a very electronegative element (note its proximity to fluorine, the most electronegative element). Therefore, the major resonance structures include the negative charge on one or more oxygen atoms, rather than on the central atom (e.g. S, P, N, etc).

Answer 2

ohh, kay, thanks, but for ICl4-, i dont understand why the extra electron is given to the Iodine instead of the chlorine. :S is that an exception?

Answer 3

Not exactly - while electronegativity is often a good indicator of which atom will receive extra electrons preferentially in a compound, there are other factors as well that I'll mention since you brought up ICl4-.

An atom can only take on extra electrons if it has empty orbitals to do so. In the case of chlorine and iodine, you are right that Cl is more electronegative than I. However, Cl does not have an empty orbital to accommodate additional electrons once it has bonded to I. I, however, has empty d sublevel orbitals into which additional electrons can be added. In fact, this is the reason why it is able to bond to four chlorine atoms, despite having only one unpaired electron in the valence shell. This also breaks the octet rule.

I guess a good rule of thumb is to determine whether there are empty orbitals available, then choose the more electronegative element of those with empty orbitals.