Final answer:
Carbon does not exist as a diatomic molecule. Common diatomic molecules include hydrogen, oxygen, nitrogen, and chlorine.
Step-by-step explanation:
The element that does not exist as a diatomic molecule in the list provided is carbon. Diatomic molecules are pairs of atoms of the same element bonded together. Some common elements that exist naturally as diatomic molecules include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), and iodine (I₂). Unlike these elements, carbon exists in several allotropic forms, such as graphite and diamond, but not commonly as a diatomic molecule.