Diatomic molecules are molecules made only of two atoms, of either the same or different chemical elements.
Two atoms are bonded together to form a unit known as a diatomic molecule.
Examples are hydrogen (H2), nitrogen (N2) and oxygen (O2), and carbon monoxide (CO). 
- ↑ Whitten, Kenneth W.; Davis, Raymond E.; Peck, M. Larry; Stanley, George G. (2010). Chemistry (9th ed.). Brooks/Cole, Cengage Learning. pp. 337–338.CS1 maint: Uses authors parameter (link)