The nitrate ion is a polyatomic ion with the empirical formula NO3- and a molecular mass of 62.0049. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identical oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a formal charge of negative one, where each oxygen carries a -2/3 charge while the nitrogen carries a +1 charge, and is commonly used as an example of resonance.