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The formal charge does not always need to equal 0 for the Lewis Structure to be favorable. If the compound has an overall charge (for example, OH^-) then the overall formal charge between all elements will be -1. What makes a Lewis Structure favorable is its ability to have the lowest formal charge numbers between all elements. To clarify, the closer the Formal Charges for each element are to zero, the more likely that Lewis Structure will be favored. I hope this helps!!
When comparing the formal charges of structures of a compound, it is clear that the structure with a formal charge closest to zero is the most stable. To calculate the formal charge it is easiest by taking the elements number of valence electrons subtracted by number of bonds and additionally subtract lone pairs (by each electron)
To add on to the previous posts, the formal charge should also be the most favorable when the central atom has a formal charge of zero. This is due to the least electronegative atom being in the center. Thus, the central atom shouldn't have charges. It is better for the atoms around the central atom to have charges. The most electronegative atoms, which should be around the central atom should have charges such as, -1 or -2.
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