Determining formal charge

[Isabel Carden 1E]

Hi!

I'm a little confused about formal charge. In the example Dr. Lavelle gave in class of a sulfate ion (SO4)-2, the individual formal charges added up to the overall charge of the ion, which is -2. When I was working on some of the workbook problems, however, this wasn't the case. For example, for the ion NO+ I found that the formal charge of O was +2 and N to be +1, which doesn't add up to the total charge of the ion (+1). Should the sum of the formal charges equal the total charge, or was this just a coincidence in the example he gave?

Thank you!

[Jericho Ansay 1C]

Yes, the sum of the formal charges should equal the total charge. When determining formal charge, you have to make sure that your Lewis structure is accurate. When I drew the Lewis structure of NO+, I had nitrogen double bonded to oxygen with one lone pair on nitrogen and two lone pairs on oxygen. This gave me a formal charge of +1 on nitrogen and 0 on oxygen, which adds up to the +1 charge of the atom.

[Mia Yamada 1J]

Hi!

When I drew out the Lewis structure, I also got that the sum of the formal charges equals the total charge of the ion, except I had nitrogen triple bonded to oxygen with one lone pair on both nitrogen and oxygen, to result in a full octet for both atoms. So, instead, I got a formal charge of 0 on nitrogen and a formal charge of +1 on oxygen, which in the end still adds up to the total +1 charge! I believe this might be the right (most stable) Lewis structure, but please correct me if I'm wrong! :)

[Aneesha_Nema_3C]

Hey! I drew out the Lewis structure so that the N and O were double bonded and N had one lone pair of electrons while O had two. Using the formal charge equation FC = V - (L + S/2), I calculated the formal charges to be:

N: FC = 5 - (2 + 4/2) = 1
O: FC = 6 - (4 + 4/2) = 0

These formal charges add up to be +1, which is the overall charge of the molecule.

[Vivian Kim 3F]

Hey, I just wanted to add that even though you draw the structure differently (for instance the resonance structures) the charges should all add up to the charge of the ion.

[Kailin Mimaki 2K]

In order to determine formal charge, use the equation: FC=V-N-B/2. Plug in the number of valence electrons for V, the number of nonbonding valence electrons for N, and the total number of electrons shared in bonds for B. Then, you would have calculated the formal charge. Hope this helped!

[Connor_Olsen_2K]

It has to add up. The formal charges should add to the overall charge every time.