Question

Why helium existed as He hydrogen exist as H2

Answer 1

In an atom, there are specific paths in which electrons revolve around the nucleus. Each shells has to attain a number of 8 to be stable (except the first shell). In the first shell there need to be only two. There are some elements called the noble gases which are stable. Helium is one of them thus it exist individually where as Hydrogen needs to share to be stable. Thus it exists as H2.