Hydrogen is a diatomic gas with two atoms (di meaning two). H2 is referred to as molecular hydrogen because hydrogen atoms are covalently joined together to form a molecule.
Because two hydrogen molecules are connected to one other by a covalent connection, hydrogen is denoted as H2.
Hydrogen has only one electron, so to complete its duet(octet in case of nitrogen) it has to bond with another hydrogen molecule to be stable and also exist.
You can remember it like this :
H = element
H2 = gas
Other diatomic gases :
O2 (oxygen)
N2 (nitrogen)
