## Formal Charges

$FC=V-(L+\frac{S}{2})$

xenamclean_1G
Posts: 51
Joined: Sat Jul 20, 2019 12:15 am

### Formal Charges

Can anyone explain how to use formal charges when drawing the resonance structure?

John Liang 2I
Posts: 61
Joined: Fri Aug 30, 2019 12:18 am

### Re: Formal Charges

Your formal charges should "make sense" when drawing resonance structures. Take your first structure and see if you can move any double bonds. Then, take the formal charge of the atoms with your resonance structure and evaluate them. Usually, they should be 0 or negative 1 and positive 1.

Alan Wu
Posts: 61
Joined: Sat Sep 14, 2019 12:16 am

### Re: Formal Charges

In a resonance structure, the formal charges for same atoms should all be equal in reality. This is because in reality, the structure is a hybrid of all the resonance structures. For example, in the SO4 2- resonance structures with 2 Os connected to S with double bonds and 2 Os connected to S with single bonds, the calculated formal charges for the Os with double bonds are 0 and the Os with single bonds are +1. However, in reality, all the of the Os should have a +1/2 formal charge because they all average out to be the same in the hybrid.

Jordan Young 2J
Posts: 66
Joined: Thu Jul 25, 2019 12:17 am
Been upvoted: 1 time

### Re: Formal Charges

Formal charge is used to determine which resonance structure is "best" or the most stable. By calculating the formal charge, you can determine which resonance structure is most likely to occur. Ideally, you want the higher electronegative atoms to have the negative charges and you want to disperse the charge on as many atoms as possible.

Junwei Sun 4I
Posts: 87
Joined: Wed Oct 02, 2019 12:16 am

### Re: Formal Charges

Formal charge for the same atom in resonance structures should be the same. For example in SO4 2-, when you draw out the lewis structure you get S as the central atom with two Os attached to it by double bonds and two Os attached to it by single bonds. No matter how you orientate the double bonds in different resonance structures, in each structure the formal charges for the central atom S and O with double bond and O will dingle bond should not change.