* HNO: This is a polyatomic molecule, meaning it contains more than two atoms.

* H2O: This is also a polyatomic molecule (water).

* Br2: This is a diatomic molecule, meaning it consists of two atoms of the same element (bromine).

* Co2: This is a polyatomic molecule (carbon dioxide).

Therefore, the only diatomic molecule in your list is Br2.