Formal Charge equation

[Katherine Chhen 3I]

I know that the formal charge equation is FC=V-(L+S/2), but my TA taught us FC=V-(L+B), with B representing the bond length, is this equation essentially the same?

[Tai Metzger 3K]

I'm not sure if their the same thing but you should probably stick to the one Dr. Lavelle gave.

[MinuChoi]

Bond length probably won't work for that.
But that form FC=V-(L+B) would work for B = number of bonds that atom has (+1 for singles, +2 for doubles), which is essentially the same thing as S/2.
It's actually easier for me personally to do FC = V - (L+B) because I can simply count the number of bonds the atom has then count them as pairs, then divide by 2.

[Jasmine Fendi 1D]

hello! the equation for formal charge may be confusing, so my TA told us that you do not have to memorize it. I think you can calculate it using the number of valence electrons from the periodic table and the number of shared electrons in the lewis structure

[Tiffany Chao 2H]

I think you should just stick with the one Dr. Lavelle gave us. The problem probably wouldn't give you bond length in the first place so you couldn't figure out the formal charge with the equation your TA gave you.

[Pegah Nasseri 1K]

It is easier to use the equation that Lavelle gave us in class for formal charge. Another quick way for calculating formal charge without using the equation is by simply knowing the number of valence electrons an element has and subtracting that by the amount of valence electrons it has in the lewis structure. For example, for the molecule CCl4 the lewis structure would have Carbon as the central atom bonded to 4 chlorines. Each bond it forms with a chlorine is a single bond. So you know that Carbon has 4 valence electrons by looking at the periodic table. In the lewis structure, Carbon forms 4 bonds which is a total of 8 electrons. You would divide 8 by 2 because the electron pairs are being shared, meaning that in the lewis structure Carbon has 4 valence electrons. Subtracting 4 by 4 would give you zero and so the formal charge is zero. In this way, you are doing the exact same thing as the equation Lavelle gave us but it is faster because you can just count the number of electrons the element has in the lewis structure (only count 1 electron if it is a single bond because it is a shared electron pair) and subtract it from the original valence electrons of the element.

[805097738]

I think you misunderstood what B represents. Your TA was probably referring to the number of bonds, which would work for the equation.

[Jessica Tran_3K]

How often are we supposed to use formal charge to check our lewis structures? Do you check it every time?

[sbottomley3a]

One way to simplify the equation that Lavelle gave us in class is to take the total number of valence electrons in your compound, and then subtract the number of 'sticks' (or bonds) and subtract the number of 'dots' (lone electrons). For me, this saves time and I always get the same formal charge that I would have gotten using Lavelle's equation.

[Jaci Glassick 2G]

I'm not exactly sure but I think the B stands for number of bonds, not the bond length. But when in doubt, just stick with the equation Dr. Lavelle gave us. Hope this helps!