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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. == Examples == Ammonium NH4+ is a cationic species. By using the vertical groups of the atoms on the periodic table it is possible to determine that each hydrogen contributes 1 electron, the nitrogen contributes 5 valence electrons, and the charge of +1 means that 1 of the contributed electrons is absent. The final total is 8 total electrons (1 × 4 + 5 − 1). Drawing the Lewis structure gives an sp3 (4 bonds) hybridized nitrogen atom surrounded by hydrogen. There are no lone pairs of electrons left. Thus, using the definition of formal charge, each hydrogen has a formal charge of zero and the nitrogen has a formal charge of . After adding up all the formal charges throughout the molecule the result is a total formal charge of +1, consistent with the charge of the molecule given in the first place. Note: The total formal charge in a molecule should be as close to zero as possible, with as few charges on the molecule as possible *Example: CO2 is a neutral molecule with 16 total valence electrons. There are three different ways to draw the Lewis structure * *Carbon single bonded to both oxygen atoms (carbon = +2, oxygens = −1 each, total formal charge = 0) * *Carbon single bonded to one oxygen and double bonded to another (carbon = +1, oxygendouble = 0, oxygensingle = −1, total formal charge = 0) * *Carbon double bonded to both oxygen atoms (carbon = 0, oxygens = 0, total formal charge =0) Even though all three structures gave us a total charge of zero, the final structure is the superior one because there are no charges in the molecule at all. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「formal charge」の詳細全文を読む スポンサード リンク
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