Hydrogen is the chemical element with symbol H and atomic number 1. It is the lightest element and occupies the first position in the periodic table. Under ordinary conditions, elemental hydrogen consists of diatomic molecules, H₂, forming a colorless, odorless gas. Hydrogen occurs extensively in water and organic compounds, while its industrial applications include chemical manufacture, petroleum refining, and energy conversion. (periodic-table.rsc.org)
Atomic structure and isotopes
A neutral hydrogen atom contains one proton and one electron. Its ground-state electron configuration is 1s¹. The three principal isotopes differ in their numbers of neutrons: protium, ¹H, has none; deuterium, ²H or D, has one; and tritium, ³H or T, has two. Protium accounts for more than 99.98% of naturally occurring hydrogen, while deuterium is stable and much less abundant. (periodic-table.rsc.org)
Tritium is radioactive, undergoing beta decay with a half-life of approximately 12.3 years. It occurs naturally in small quantities and is also produced through nuclear reactions. Because isotopes share the same electronic structure, their chemical behavior is broadly similar, although their different masses affect physical properties and reaction rates. (nrc.gov)
Hydrogen’s simple atomic structure makes it a central system in quantum mechanics. The nonrelativistic Schrödinger equation for an isolated hydrogen atom admits exact solutions within the idealized Coulomb model. Measurements of its spectral lines and energy-level structure provide precise comparisons between experiment and atomic theory; hydrogen spectroscopy also distinguishes isotopes through small shifts in their spectra. (dlmf.nist.gov)
Discovery and cosmic occurrence
Flammable gas released by reactions between metals and acids was observed before hydrogen was recognized as a distinct substance. In 1766, Henry Cavendish collected and characterized it as different from other gases. Antoine Lavoisier subsequently gave it a name meaning “water-former,” reflecting the production of water when hydrogen burns in oxygen. (periodic-table.rsc.org)
Hydrogen is the most abundant element in the universe. The early universe following the Big Bang contained approximately three-quarters hydrogen and one-quarter helium by mass, with only traces of other light elements. This proportion concerns ordinary elemental matter, not the universe’s total contents including dark matter and dark energy. (imagine.gsfc.nasa.gov)
Hydrogen is a major constituent of stars, where nuclear fusion converts hydrogen nuclei into helium and releases energy. This process powers the Sun and other hydrogen-burning stars. On Earth, hydrogen is chiefly encountered chemically bound in water, hydrocarbons, and numerous other compounds rather than as free atmospheric gas. (ntrs.nasa.gov)
Physical and chemical properties
Hydrogen has the lowest density of any gas. At approximately atmospheric pressure, it melts near 14 K and boils near 20.3 K, so liquid hydrogen requires extremely low temperatures. These properties distinguish its storage and handling from those of ordinary liquid fuels. (periodic-table.rsc.org)
In H₂, the atoms share a pair of electrons through a covalent bond. Hydrogen commonly has an oxidation state of +1 in compounds with more electronegative elements and −1 in ionic metal hydrides. Such hydrides contain the H⁻ ion and can react vigorously with water to release hydrogen gas. Hydrogen also combines with oxygen to form water; an ignited mixture of the two gases can react explosively. (openstax.org)
Hydrogen is central to acid–base chemistry, in which proton transfer provides a major framework for describing reactions. Separately, a hydrogen bond is an attraction involving hydrogen covalently bonded to an electronegative atom and another suitable atom. It is distinct from the bond within H₂. Hydrogen bonding helps explain water’s unusually high boiling point relative to comparable small molecules. (openstax.org)
Production and industrial uses
Hydrogen can be manufactured from several feedstocks. Natural-gas reforming reacts hydrocarbons with steam to produce hydrogen-containing gas; subsequent processing increases hydrogen yield and separates the product. These pathways also generate carbon dioxide. Electrolysis instead uses electricity to split water into hydrogen and oxygen. Its associated emissions depend substantially on how the electricity is generated. (openstax.org)
A major use is the manufacture of ammonia through the Haber process, which combines hydrogen with nitrogen. Hydrogen is also used in petroleum refining to remove sulfur, in producing methanol and other chemical intermediates, and in hydrogenating unsaturated oils. These applications employ hydrogen as a chemical reactant rather than simply as a fuel. (periodic-table.rsc.org)
Energy conversion, storage, and safety
Manufactured hydrogen functions as an energy carrier: energy is expended to produce it and can later be recovered. A fuel cell converts hydrogen’s chemical energy directly into electricity through electrochemical reactions, producing water and heat when supplied with hydrogen and oxygen. Environmental accounting therefore extends beyond the point of use to production and delivery. (energy.gov)
Hydrogen’s low volumetric energy density creates storage challenges. Options include compressed gas, cryogenic liquid, and materials that bind or adsorb hydrogen. Storage systems must accommodate pressure, temperature, and material compatibility. (hydrogen.energy.gov)
Hydrogen is highly flammable, and its lack of color and odor complicates leak detection. It can also cause hydrogen embrittlement in susceptible metals, altering their mechanical properties and increasing the likelihood of cracking. These characteristics influence the design of tanks, pipelines, detectors, and other hydrogen-handling equipment. (energy.gov)