How is a hydrogen-1 atom different form a hydrogen-2 atom? How are they similar?

If you’re referring to H vs H2,

H is usually found in ion form as H+

H2 is the designation for pure hydrogen gas, as it is diatomic and can only exist on its own in nature if it’s bonded to another H atom.

In this case, H is an ion and H2 is hydrogen gas

The way you wrote them out, though, indicates that they could be something different. Maybe isotopes? In this case the 1 and 2 are indicative of the amount of neutrons that the atom has.