Question

why we write hydrogen gas "h2"why we can't write "H"​

Answer 1

The atom is H, but the element is H . In chemical formulas involving molecules, you always work with entire molecules and never with unbound atoms. The element hydrogen only occurs as molecules of the gas H , there is no molecule that has only one hydrogen atom. ... So, wheneverhydrogen is mentioned, it's always H2

Answer 2

We generally don't write hydrogen as "H" in the atmosphere it is found in "H2" and not in "H". It is similar to that of oxygen.