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Finding the formal charges helps you determine how favorable the resonance structure is. The best resonance structures have formal charges on each atom as close to zero as possible, with any negative formal charges on more electronegative atoms and any positive formal charges on less electronegative atoms. The sum of all the formal charges also needs to equal the charge on the compound, ie if the compound is an anion with a charge of -1, all the FC added together must equal -1.
It also helps you keep track of the electrons and can help you predict the reactivity of the molecule. It assumes that all electrons are shared equally so you don't have to worry about bond polarity contributing to differences in potential. It'll help more when we do coordinate covalent complexes later on I think.
Dr. Lavelle said in lecture that the formal charge of an atom indicates a gain or loss of electrons when forming a covalent bond. Calculating formal charges is helpful in drawing the best possible Lewis structures and making sure that the overall charge of the molecule is represented in the Lewis structure. Using the example he did in class, the molecule for Sulfate has an overall charge of -2. This means that when drawing the Lewis structure, the formal charges of each atom must add up to -2.
Matt F wrote:Anna Chen 4L wrote:The resonance structure with the lowest formal change is the most stable.
I thought the structure with its formal charge = 0 or closest to 0 was the most stable?
You are correct. The structure with the formal charge closest to zero is most stable since that would result in it having no charge of the molecule.
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