A heteronuclear molecule is a molecule composed of atoms of more than one chemical element.^[1][2] For example, a molecule of water (H₂O) is heteronuclear because it has atoms of two different elements, hydrogen (H) and oxygen (O).

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Similarly, a heteronuclear ion is an ion that contains atoms of more than one chemical element. For example, the carbonate ion (CO²⁻
₃) is heteronuclear because it has atoms of carbon (C) and oxygen (O). The lightest heteronuclear ion is the helium hydride ion (HeH⁺). This is in contrast to a homonuclear ion, which contains all the same kind of atom, such as the dihydrogen cation, or atomic ions that only contain one atom such as the hydrogen anion (H⁻).