 There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei . In the orthohydrogen form , the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 ( 1 ⁄ 2 + 1 ⁄ 2 ) ; in the parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 ( 1 ⁄ 2 – 1 ⁄ 2 ) . At standard temperature and pressure , hydrogen gas contains about 25 % of the para form and 75 % of the ortho form , also known as the " normal form " . The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature , but because the ortho form is an excited state and has a higher energy than the para form , it is unstable and cannot be purified . At very low temperatures , the equilibrium state is composed almost exclusively of the para form . The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities , as discussed more fully in spin isomers of hydrogen . The ortho / para distinction also occurs in other hydrogen @-@ containing molecules or functional groups , such as water and methylene , but is of little significance for their thermal properties . 
