A formal charge is the amount of electrons shared between atoms within a molecule. When a chemical bond occurs between two or more atoms, the valence electrons are shared between the atoms. These are the outermost layer of electrons of an atom. Essentially, the formal charge is designed to determine the number of electrons each atom contributes to the molecule. This union is called a covalent bond.
When calculating this concept, its important to understand the correct structure of the molecule. This can be accomplished visually, by using the method known as the Lewis structure. The Lewis structure is a defined set of parameters that enables scientists and researchers to be able to diagram how atoms of a molecule bond together. One of the primary benefits of using the Lewis structure is that it identifies where the electrons of a covalent bond are located, making it easier to determine the formal result.
The exact formal charge formula is not so much a mathematical equation, but more of a generality. Formal charge is first identified by finding the number of valence electrons of a given atom when it is in its ground state, meaning that it has no anomalous charge. Within each molecule, there will be a certain number of electrons that are not bound to other atoms. These should be subtracted from the amount of valence electrons. The final step is to find out the number of electrons throughout other covalent bonds within the molecule, these should then be subtracted from the remaining number, giving the correct formal charge.
The principal difference between the this idea of molecular structure and the oxidation state is the fact that electronegativity is not taken into account with a formal charge. Electronegativity is important in order to understand how strong the attraction can be between different atoms. This helps determine the oxidation level of a molecule. However, with the formal charge, the purpose is merely to determine what charge the entire molecule maintains as a unit, rather than its likelihood to bond with other elements.
By determining this principle, it is possible to determine if a particular molecule has retained all or lost some electrons. A rule of thumb in formal charge chemistry is the fact that the number of electrons is usually very close to zero. Sometimes, there is a negative number or a positive number, but the equation is set up to help identify the number of missing electrons. For example, if a particular molecule formulates into a result of zero, than no electrons are missing from the original elements. If it shows a negative one, however, then one of the electrons was lost.