Question

helium is considered monoatomic but hydroges is considered diatomic molecule.why?

Answer 1

For the same reason that the other noble gases are not diatomic. He's outer valence shell is maxed-out with electrons and He is happy to be by itself. Here "happy" means that the state of lowest potential energy for He is to be monoatomic. For this Eric Karmen sings, "All by Myself" and we feel sorry for lonely Helium.

Answer 2

helium cant make bond with any other element bcz it is innert
on the other hand hydrogen reacts with itself to form diatomic molecule