In chemistry, a formal charge (F.C. or ), in the covalent view of chemical bonding, is the hypothetical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity. In simple terms, formal charge is the difference between the number of valence electrons of an atom in a neutral free state and the number assigned to that atom in a Lewis structure. When determining the best Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge on each of the atoms is as close to zero as possible. The formal charge of any atom in a molecule can be calculated by the following equation: where is the number of valence electrons of the neutral atom in isolation (in its ground state); is the number of non-bonding valence electrons assigned to this atom in the Lewis structure of the molecule; and is the total number of electrons shared in bonds with other atoms in the molecule. Usage conventions In organic chemistry convention, formal charges are an essential feature of a correctly rendered Lewis–Kekulé structure, and a structure omitting nonzero formal charges is considered incorrect, or at least, incomplete. In contrast, this convention is not followed in inorganic chemistry. Formal charges are drawn in close proximity to the atom bearing the charge. They may or may not be enclosed in a circle for clarity. On the other hand, many workers in organometallic and a majority of workers in coordination chemistry will omit formal charges, unless they are needed for emphasis, or they are needed to make a particular point. Instead a top-right corner ⌝ will be drawn following the covalently-bound, charged entity, in turn followed immediately by the overall charge. This difference in practice stems from the relatively straightforward assignment of bond order, valence electron count, and hence, formal charge for compounds only containing main-group elements (though oligomeric compounds like organolithium reagents and enolates tend to be depicted in an oversimplified and idealized manner), while there are genuine uncertainties, ambiguities, and outright disagreements when these assignments are attempted for transition-metal complexes. Examples Example: CO2 is a neutral molecule with 16 total valence electrons. There are different ways to draw the Lewis structure Carbon single bonded to both oxygen atoms (carbon = +2, oxygens = −1 each, total formal charge = 0)