We're going to start building structures of molecules using the Lewis model. Part of drawing structures will be identifying which atoms bear a charge. Formal charge is a formalism that compares the number of electrons that an atom in a molecule possesses to the number of electrons in the free, neutral atom. If an atom in a molecule has the same number of valence electrons as the free, neutral atom, it has no formal charge. If it has more valence electrons than the neutral atom, it bears a negative formal charge; if it has fewer, it bears a positive formal charge. From time to time you’ll come across molecules for which you can draw multiple valid Lewis structures. One strategy for deciding which Lewis structure is “best” is to pick the structure that has the fewest atoms with formal charges.
See the video below to learn how to determine formal charge.