Nesecito la respuesta?

Halogens only are the elements which can form diatomic molecules joined by a single covalent bond.

Explanation:

Hydrogen is an atom which has the ability to form monoatomic, diatomic atoms.

For example, hydrogen can exists as H, and molecule.

As valency of halogen is 7 and in order to attain stability it needs to acquire one electron from a donor atom. Hence, halogens tend to form a single covalent bond by combining with same or different halogen.

For example, in there will exist a single bond between both the bromine atoms.

Therefore, we can conclude that halogens only are the elements which can form diatomic molecules joined by a single covalent bond.