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Response Details:
I always had a strange time calculating formal charges but the wikipedia alternative method helped me, hope it helps you understand it better: (wikipedia.org)
Although the formula given above is correct, it is often unwieldy
and inefficient to use. A much quicker and still accurate method is to
do the following:
- Draw a circle around the atom for which the formal charge is requested (as with carbon dioxide, below)
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- Count up the number of electron in the atom's "circle." Since the
circle cuts the covalent bond "in half," each covalent bond counts as
one electron instead of two.
- Subtract the number of electrons in the circle from the group
number of the element (the roman numeral from the older system of group
numbering, NOT the IUPAC 1-18 system) to determine the formal charge.
(aka: old group number minus electrons in circle)
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- The formal charges computed for the remaining atoms in this Lewis structure of carbon dioxide are shown below.
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Again, this method is just as accurate as the one cited above, but
is much easier to use. It is important to keep in mind that formal
charges are just that-formal, in the sense that this system is a
formalism. Atoms in molecules do not have "signs around their necks"
indicating their charge. The formal charge system is just a method to
keep track of all of the valence electrons that each atom brings with
it when the molecule is formed.
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