Do full valence shells always result in a formal charge of zero?

Question

Isabella Acosta · Accepted Answer

The simple answer is no. For instance, the halide ions bear a formal negative charge with full valence shells.

An atom's formal charge is determined by the ratio of protons in its nucleus to the total number of electrons surrounding it (or shared by it!). An atom is considered neutral if the ratio is equal; an excess or deficiency of electrons produces a formal charge. A good place to start would be the ozone molecule, #O_3#. This is certainly a neutral molecule, but its common valence representation features a neutral #O# atom bound to a formally positively charged #O# atom, bound to a formal negatively charged #O# atom, i.e. #O=^+O-O^-#. The central atom bears an undepicted lone pair. Why does the central #O# atom bear a formal positive charge?

Jacob Anderson · Answer

undefined No, full valence shells do not always result in a formal charge of zero. Formal charge is calculated by subtracting the number of lone pair electrons and half the number of bonding electrons from the total number of valence electrons in an atom. Even if an atom has a full valence shell, it can still have a formal charge if it is involved in bonding where the electrons are not evenly shared. Therefore, the formal charge of an atom depends on its bonding arrangement within a molecule or ion.