Final answer:
Chlorine (Cl) has a higher first ionization energy compared to the diatomic molecule.
Step-by-step explanation:
The element with the highest first ionization energy compared to the diatomic molecule is Chlorine (Cl). The first ionization energy is the energy required to remove the first electron from an atom or molecule. Chlorine has a higher first ionization energy compared to the diatomic molecule because it is easier to remove an electron from a single atom than from a bonded pair of atoms.