H2molecule

H2, or molecular hydrogen, is the diatomic molecule consisting of two hydrogen atoms bonded together by a covalent single bond. It is the simplest and one of the most abundant molecular species in the universe, playing a major role in stellar and interstellar chemistry as well as in terrestrial processes.

The H–H bond arises from the sharing of a pair of electrons between the two 1s atomic orbitals, forming a sigma bond. The bond length is about 74 picometers, and the bond energy is approximately 436 kJ per mole. This simple bond gives H2 a formation energy and a set of vibrational and rotational energy levels that can be probed by spectroscopy.

In standard conditions, hydrogen is a colorless, odorless, and non-toxic gas with a very low density. It is highly flammable and forms explosive mixtures with air. Hydrogen can be liquefied at very low temperatures (its boiling point is 20.28 K at 1 atm) and is widely used as a cryogenic liquid and a rocket fuel. It also serves as a reducing agent and a feedstock in chemical industries, for example in ammonia synthesis and hydrocarbon processing.

Hydrogen exists in nuclear spin isomers known as ortho-H2 and para-H2, distinguished by the relative spins of the two protons. At room temperature the equilibrium mixture is about 3:1 in favor of ortho-H2; cooling shifts the balance toward para-H2. Conversion between forms can be catalyzed by surfaces and affects thermodynamic properties at low temperatures.

H2 is a fundamental subject of study in chemistry and physics, providing a benchmark for quantum chemical calculations and a key component in astrophysical environments.