In chemistry, a formal charge (FC) is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity.
The formal charge of any atom in a molecule can be calculated by the following equation:
Where V is the number of valence electrons of the atom in isolation (atom in ground state); N is the number of non-bonding valence electrons on this atom in the molecule; and B is the total number of electrons shared in covalent bonds with other atoms in the molecule. There are two electrons shared per single covalent bond.
When determining the correct Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge (without sign) on each of the atoms is minimized.
Formal charge is a test to determine the efficiency of electron distribution of a molecule. This is significant when drawing structures.
Examples:
- Carbon in methane: FC = 4 - 0 - (8÷2) = 0
- Nitrogen in NO2-: FC = 5 - 2 - (6÷2) = 0
- double bonded oxygen in NO2-: FC = 6 - 4 - (4÷2) = 0
- single bonded oxygen in NO2-: FC = 6 - 6 - (2÷2) = -1
An alternative method for assigning charge to an atom taking into account electronegativity is by oxidation number. Other related concepts are valence, which counts the number of electrons that an atom uses in bonding, and coordination number, the number of atoms bonded to the atom of interest.
Read more about Formal Charge: Examples, Alternative Method, Formal Charge Compared To Oxidation State
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