A molecule consisting of two hydrogen atoms bonded together, the common form of hydrogen gas.The hydrogen molecule (H₂) is the simplest neutral molecule, consisting of two hydrogen atoms joined by a covalent bond. It is the most abundant molecule in the universe, making up a large fraction of the gas in interstellar clouds and the atmospheres of gas giants. Its simplicity makes it a fundamental testbed for molecular quantum mechanics and spectroscopy.
The quantum-mechanical treatment of the hydrogen molecule was first carried out by Walter Heitler and Fritz London in 1927, marking the birth of quantum chemistry.
The two hydrogen atoms share a pair of electrons, forming a bond that arises from the constructive interference of the atomic wavefunctions. The molecule's energy levels are determined by electronic, vibrational, and rotational degrees of freedom, each quantized. The electronic ground state has a binding energy of about 4.5 eV, and transitions between rotational and vibrational levels produce characteristic spectra in the infrared and microwave regions.
Hydrogen molecules are used in fuel cells, as a rocket propellant, and in industrial processes such as ammonia synthesis; their spectral lines are used to study interstellar gas.
The hydrogen molecule has two distinct forms — ortho-hydrogen and para-hydrogen — which differ in the relative spin orientation of the two protons and have slightly different thermal properties.
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