Oxygen can form double covalent bonds because it has six valence electrons and can share two pairs of electrons with another atom. Chlorine, on the other hand, has seven valence electrons and would need to share three pairs of electrons to form a double bond, which is not possible. Double covalent bonds are most common between atoms with four or fewer valence electrons because these atoms can easily share two pairs of electrons. Atoms with five or more valence electrons can also form double bonds, but they are less common. Chlorine is considered one of the halogens, which are characterized by their single bond or electron pair and seven valence electrons.