How do you calculate the formal charge of NO3-?
The formal charge of the nitrate anion is of course
Formal charge is by definition a formalism; it has no physical reality, but may nevertheless be useful for calculation.
We can write the Lewis representation of the nitrate anion as,
Compare this with ammonia, a neutral molecule. We write
So back to nitrate: nitrogen has dibs on only 6 electrons, and therefore bears a formal positive charge. The doubly bound oxygen has a share in or owns 8 electrons, and so is depicted as neutral. The singly bound oxygen atoms have 9 electrons associated with them, and so each bears a negative charge. The overall charge on the nitrate ion is of course