Why does the sume of the formal charge in a neutral molecule always add up to zero?
The sum of the internal charges for all molecules must be zero, or the molecule would not be a molecule at all but an ion. Likewise ions cannot exist independently. If a cation exists, anions with equal but opposite electrical charges must be present. For example, if you have common table salt dissolved in water (or melted), for every Na+ cation there must exist a Cl- anion to balance its charge. Think about the consequences if this were not true. Every time you tried to salt your veggies, sparks would fly all over the place. So no matter what compound you pick, this rule of electrical neutrality must apply.
