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The equation for formal charge that I use is the one given during class, which is FC= V - (L + S/2) where V is the number of valence electrons, L is the number of lone pair electrons, and S is the bonds shared. Unfortunately, I do not know, which equation you used in discussion to find the formal charge, but I am assuming that it is probably the same thing as the equation above and maybe just written in a different way. You should use this equation to find the formal charges of atoms in order to see which structure has the lowest energy.
Another way to determine formal charge is through a variation of the equation FC=V-(L+S/2). One way to think about it is to use number of bonds instead of S/2. The way that I usually calculate formal charge is by doing FC=V-(L+ #of bonds). S/2 is the same as simply counting each bond as 1 (since you assume each atom in the molecule shares half the electrons in the bond), so you can just add up the lone pairs and the number of bonds around an atom and subtract it from the number of valence electrons associated with the atom to find formal charge. In general, you do need to use either the first equation or the variation of the equation to solve for the formal charge on each atom.
Arianna Perea 3H wrote:Does FC determine where we make double bonds?
Yes, it does! The FC equation needs to know the number of paired and unpaired electrons, and the most stable structures are those with a net charge of zero. FC can determine where double bonds will be when calculated.
You should keep in mind formal charges when drawing lewis structures since they give you the most stable structure of the molecule. To calculate it you can just take the valance electrons the element has and subtract the number of lone pair electrons (dots) and the number of bonds the element forms (lines).
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