Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas?

Hydrogen gas is diatomic in nature($H_2$) while helium gas is monoatomic (He). As a result, the number of atoms in one mole of hydrogen(2$\times$N_A )is expected to be double when compared to the number of atoms in one mole of helium($N_A$).