Why do chlorine molecules contain two chlorine atoms?

Chlorine atoms exist as diatomic molecules (${\text{Cl}}_2$) because this configuration allows them to achieve a more stable electron arrangement.

As a member of the halogen elements, each chlorine atom possesses seven valence electrons in its outermost shell. According to the octet rule, atoms tend to gain, lose, or share electrons to attain a stable configuration of eight electrons, similar to that of the noble gases.12

By forming a covalent bond with another chlorine atom, each atom shares one of its electrons to complete its valence shell. This sharing of an electron pair results in a lower total energy state for the molecule compared to the individual, separated atoms, thereby increasing stability. Consequently, at room temperature and atmospheric pressure, chlorine exists in its free state as a diatomic molecule.314