Alternative Method
The following is equivalent:
- Draw a circle around the atom for which the formal charge is requested (as with carbon dioxide, below)
- Count up the number of electrons 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.
- The formal charges computed for the remaining atoms in this Lewis structure of carbon dioxide are shown below.
It is important to keep in mind that formal charges are just that-formal, in the sense that this system is a formalism. 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.
Read more about this topic: Formal Charge
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