H2

Hydrogen, with the chemical formula H2, is the diatomic molecule consisting of two hydrogen atoms bonded together. It has a molecular weight of about 2.016 g/mol and forms a linear, nonpolar molecule. The bond length is approximately 74 picometers and the bond energy is around 436 kJ/mol. At standard conditions, H2 is a colorless, odorless, and tasteless gas with very low density and high diffusivity. It is highly flammable and forms explosive mixtures with air.

Hydrogen is the most abundant element in the universe by mass, making up the majority of ordinary matter in stars and gas giant planets. On Earth, it occurs mainly in combination with other elements, such as water (H2O) and hydrocarbons. Hydrogen has three known isotopes: protium (1H), deuterium (2H), and tritium (3H). The two spin isomers of H2, orthohydrogen and parahydrogen, exist in a temperature-dependent ratio and can affect physical properties.

Industrial production of hydrogen is dominated by steam methane reforming and by electrolysis of water. It is a versatile chemical feedstock and reducing agent, essential in ammonia synthesis and various hydrogenation reactions. In energy applications, H2 is used as a fuel in internal combustion engines, in hydrogen fuel cells, and as a rocket propellant when combined with liquid oxygen. Liquid hydrogen is also used as a cryogenic coolant in some applications.

Safety and environmental aspects are notable: hydrogen’s wide flammability range and rapid diffusion require careful handling to prevent leaks and ignition. When burned or used in fuel cells, hydrogen produces only water as a byproduct, making it a potential clean energy carrier, though its production and storage present technical and safety challenges. The name hydrogen derives from Greek roots meaning “water-forming,” reflecting its role in producing water when combusted.