Formal charge

The formal charge of an atom is the charge (+ or -) assigned to that atom assuming that all the electron pairs that make up the covalent bonds are shared equally between the atoms. If the total number of electrons around an atom is higher than the number of protons in the nucleus then that atom will be negatively charged, whereas if the total number of electrons around an atom is lower than the number of protons in the nucleus then that atom will be positively charged.

The formal charge, FC, of an atom can be calculated using this formula:

where V is the number of valence electrons of the neutral atom, N is the number of non-bonding valence electrons (lone-pairs) the atom has in this molecule and B is the total number of electrons shared in bonds to other atoms in this molecule.