Question

Which is a diatomic molecule? A. Ar B. CO C. CO2 D. NaCl

Answer 1

Answer: B. CO

Explanation:

Diatomic molecules are those that are formed by two atoms of the same chemical element (homonuclear diatomic molecule) or different chemical element (heteronuclear diatomic molecule).  

In this sense, oxygen is a homonuclear diatomic molecule because it is formed by two atoms of the same element ($O_{2}$) and Carbon monoxide ($CO$) is heteronuclear diatomic molecule.

Sodium Chloride $NaCl$ is not a diatomic molecule because is a product of ionization, but it can be diatomic in its gas phase with a polar covalent bond.