Write a short note on allotropy of hydrogen.
Or
How many allotropes of dihydrogen are known? What is their importance?

The hydrogen molecule occurs in two forms known as ortho and para hydrogen. The nuclei of two atoms are spinning.
(i) When the spins of the nuclei are in the same direction (parallel spin), the molecule is known as ortho-hydrogen.

(ii) If the spins are opposite (anti-parallel spins), the molecule is para hydrogen.
Para-hydrogen has a lower internal energy and is more stable than ortho-hydrogen. Hydrogen at room temperature is an equilibrium mixture of 75% ortho-hydrogen and 25% para hydrogen. At 25K. a sample of ordinary contains 99% para and 1% of ortho form.