HOW TO DETERMINE FORMAL CHARGES
Carbon is in group 14 and has four valence electrons, thus it requires four more electrons to form an octet. It can do this by forming four covalent bonds, as each covalent bond adds one electron. The formal charge on the carbon atom is zero since each covalent bond contains two shared electrons with an electron from each atom. If the C atom only has three covalent bonds, then it will only have seven electrons (four valence plus one extra from each covalent bond) so we need to add one more electron to the C atom to complete its octet. This gives us a carbanion, e.g. [CH3]–, with one lone pair and a formal charge of –1. The same will apply for the rest of group 14.
Nitrogen, with five valance electrons, needs three covalent bonds to achieve an octet and it will then have a formal charge of zero. Like C, if it is missing one bond, it will require an extra electron and will have a formal charge of –1, e.g. amide, [NH2]–. If it has an extra bond (four bonds in total), it needs to lose an electron and so it will have a charge of +1, e.g. ammonium, [NH4]+. Similarly for other members of group 15.
Continuing with the trends of C and N, oxygen has six valence electrons and will form two covalent bonds if it is going to have a formal charge of zero (e.g. water, H2O); losing one bond gives a –1 formal charge (e.g. hydroxide, OH–) and losing both bonds gives a –2 formal charge (oxide, [O]2–). The halogens require one bond (e.g. F2) and will form a halide (e.g. F–) if they have no bonds.
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Note that boron, in group 13, with only three valence electrons, will have a –1 formal charge with four bonds.
The table below gives the formal charge for the elements of each group based on the formal charge.
Don’t try to remember the table, only the number of bonds that each group needs for zero formal charge.
Remember the compounds CH4, NH3, H2O and HF. Then one less bond is –1 and one more bond is +1, and the rules apply the same for each element in the same group.
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Formal charge practice problems are available below.
