Question

H2 is a diatomic element give reason

Answer 1

H2 contains two bonded atoms of hydrogen. So, it is a diatomic compound (not element).

Answer 2

 hydrogen, a diatomic element, cannot be by itself.  It cannot exist as simply H.  Hydrogen is so reactive that when isolated from everything but hydrogen it will combine into a diatomic (two atom) molecule.  Therefore, hydrogen gas, that is sometimes used as a fuel, exists as H2.