A diatomic molecule is one that forms a bond between two of the same molecule. The main diatomic molecules are Hydrogen, Oxygen, Nitrogen, Fluorine, Chlorine, Bromine, and Iodine. There are many reasons in quantum mechanics for this answer, but the simplest way to think about this is that the atoms want to have an outer shell of eight electrons, except for hydrogen, which wants to have two. Hydrogen, nitrogen, and chlorine can all bond with another atom of the same type to satisfy their full outer shell. Copper, on the other hand, does not form with one other copper to form Cu2, so the answer is D. Copper.
