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I think it depends on the electronegativity of the atom. For example in SO3-, the O will more likely take on the negative charge because it is more electronegative than S and hence will draw electrons more strongly to itself compared to S.
Last edited by RachaelKoh3A on Wed Nov 07, 2018 1:08 pm, edited 1 time in total.
I agree with Rachael. It depends on the electronegativity of the atom. Whatever element is the most electronegative in the compound should have the charge. It doesn't have to be specifically the central atom or the outer one.
Obviously there are exceptions, but I think that it is better to have no formal charge on the central atom in order to have the Lewis Structure of the lowest energy and most stability. Also, if you add up the formal charges of all the elements it should equal the charge of the compound.
I agree that the formal charge should be close to 0 on the central atom and if there are any charges they should be on the outer elements. It is also important to look at electronegativity, but the least electronegative element is more likely to be the central atom, so this follows the pattern that charge in on outer elements.
To add to the previous replies, the most stable structure would be the one with all of the atoms having a formal charge of zero. Usually we want the central atom to have a formal charge of zero, and if the whole molecule has a charge on it, then the outer atoms could have a formal charge that doesn't equal zero...either being positive or negative.
Like others have said, with formal charge your most stable Lewis structure is when they all have a FC of 0, but if that isn't possible you usually want your central atom to have a formal charge of 0 for it to be stable.
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