When two hydrogen molecules combine to form a diatomic molecule, their electron always spin in opposite direction according to Pauli’s exclusion principle. When one electron spins in clockwise direction, another spins in anticlockwise direction and vice versa. But their nucleus may spin in same direction or different direction. When nuclei of a hydrogen molecule spin in same direction, then the molecular hydrogen is called ortho hydrogen. When the nuclei spin in opposite directions, then the molecular hydrogen is said to be para hydrogen. So, ortho and para hydrogen are the two nuclear spin isomers of molecular hydrogen.
At room temperature, ordinary hydrogen consists of 75% ortho form and 25% para form. One lowering the temperature, ortho form decreases and para form increases. At 25 K, the sample of hydrogen consists of 99% ortho form and 1% para form. The para hydrogen can be converted into ortho hydrogen by the following ways.
– Collision of para hydrogen with atomic hydrogen
– heating para hydrogen to 800oC or more
– Passing the electric discharge.
– Mixing paramagnetic molecules like O2, NO2 etc.
Ortho and para hydrogen have similar chemical properties but differ in physical properties such as boiling point melting point, thermal conductivity, specific heat capacity, internal energy etc.
Isotopes of hydrogen
|Ortho hydrogen||Para hydrogen|
|When nuclei of a hydrogen molecule spin in same direction , then the molecular hydrogen is called ortho hydrogen.||When nuclei of a hydrogen molecule spin in different directions, then the molecular hydrogen is called para hydrogen.|
|At room temperature, ordinary hydrogen consists of 75% ortho hydrogen.||At room temperature, ordinary hydrogen consists of 25% ortho hydrogen.|
|Ortho form increases with the increase in temperature.||Ortho form decreases with the decrease in temperature.|
|It is more stable.||It is less stable.|