Definition of Molecular Ion
A molecular ion is an ion that is formed as a result of the removal of one or more electrons from a molecule. This ion is formed without the fragmentation of the structure of the molecule. When an electron is removed, it gives rise to the formation of a positive ion, for example, NH4+ whereas when an electron is added into the molecule it tends to form a negative molecular ion, for example, OH-, NO2-1, etc.
The mass of this new ion either positive or negative is consistent with the sum of the relative masses of the most abundantly naturally found isotopes of the number of atoms that have made up the molecule.