Formal charge

The rule or formula for assigning formal charge to atoms in Lewis structures is the following:

Formal charge = number of valence electrons - (number of lone-pair electrons + 1/2 number of bonding electrons)

Note that "lone pair electrons" are also known as "nonbonding pairs" or "unshared pairs".

Another rule that is very important to bear in mind is that the sum of formal charges of the Lewis structure of a molecule or ion must be equal to the net charge on the molecule or ion. If this is not the case, there is a mistake either in the formal charge assignments or in the Lewis structure (probably the wrong number of valence electrons).

Patterns of formal charge

Although we can easily calculate formal charge according to the formula above, it is helpful to be able to recognize patterns for selected elements. For example, carbon with four covalent bonds and no lone pairs has a formal charge of zero. Similarly, nitrogen with three covalent bonds and one lone pair and oxygen with two covalent bonds and two lone pairs both have formal charge of zero. Furthermore, for any element, converting a lone pair into a covalent bond changes the formal charge by plus one. Converting a covalent bond into a lone pair changes the formal charge by minus one.