Hydrogen

Did You Know?

• The H₃⁺ ion, known as protonated molecular hydrogen or the trihydrogen cation, is common in the universe and has a triangular structure.
• Neutral hydrogen in the interstellar medium is called H I, and ionized hydrogen is called H II.
• The 21-centimeter hydrogen line, detected to probe primordial hydrogen, originates from neutral hydrogen in the universe.

Properties

The electron in a hydrogen atom has a ground state energy of -13.6 electronvolts, which matches an ultraviolet photon with a wavelength of about 91 nanometers. Energy levels are labeled with quantum numbers, starting with n=1 for the ground state. When an electron drops from a higher to a lower energy level, it emits light in what is called the hydrogen spectral series. Each level splits into four hyperfine levels due to spin interactions between the electron and proton.

Precise measurements of these energy levels help define physical constants, and quantum theory breaks them down into nine contributions, with the Dirac equation eigenvalue being the largest, plus terms like relativistic recoil, self-energy, and vacuum polarization. IUPAC names the neutral atom hydrogen, the positive ion hydron (H+), and the negative ion hydride (H-). The term proton is often used for the positive ion, but it strictly applies only to the cation of the most common isotope.

Hydrogen has three natural isotopes: protium (¹H, over 99.98% abundant, with just a proton), deuterium (²H, with one proton and one neutron), and tritium (³H). Deuterium is stable and non-radioactive; water containing it is called heavy water, used as a neutron moderator and coolant in nuclear reactors, and also as a potential fusion fuel.

History

In 1671, Robert Boyle discovered and described the reaction between iron filings and dilute acids that produces hydrogen gas, though he did not note its flammability. In 1766, Henry Cavendish recognized hydrogen gas as a discrete substance, naming it 'inflammable air' and speculating it was identical to phlogiston; in 1781 he found the gas produces water when burned, and he is usually credited with discovering hydrogen as an element. By 1806 hydrogen was used to fill balloons, and François Isaac de Rivaz built the first internal combustion engine powered by a hydrogen-oxygen mixture that same year.

Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819, and the Davy lamp and limelight were invented in 1823. Hydrogen was first liquefied by James Dewar in 1898 using regenerative cooling and his vacuum flask, and he produced solid hydrogen the next year. James Clerk Maxwell observed that the specific heat capacity of H₂ departs from that of a diatomic gas below room temperature, a quantum effect arising from widely spaced rotational energy levels due to hydrogen's low mass.

Production and storage

Most hydrogen today comes from fossil fuels; less than one percent is made with low-emission methods as of 2025. The three main commercial production routes are steam reforming (often paired with water-gas shift), partial oxidation of hydrocarbons, and water electrolysis. In steam methane reforming, water and methane react at 1000 to 1400 Kelvin to produce carbon monoxide and hydrogen. Making one tonne of hydrogen this way releases carbon dioxide.

Although the reaction works better at low pressure, it is run at around 2.0 megapascals because high-pressure hydrogen sells well and pressure swing adsorption systems function more efficiently at higher pressures. The resulting gas mixture, called synthesis gas, is frequently used directly to make methanol and other chemicals. Additional hydrogen can be extracted from steam by reacting carbon monoxide with an iron oxide catalyst in the water-gas shift reaction.

Some industrial processes, like ammonia production from natural gas, generate and consume hydrogen without isolating it. Electrolysis of water is a straightforward method; commercial electrolyzers use nickel catalysts in strong alkaline solutions. Platinum works better but costs more. Hydrogen made via electrolysis powered by renewable energy is known as green hydrogen.

Applications

Large quantities of H₂ are used in upgrading fossil fuels, including hydrodesulfurization and hydrocracking, which are forms of hydrogenolysis that cleave bonds by hydrogen. Hydrogenation, the addition of H₂ to substrates, is done on a large scale; the Haber process hydrogenates nitrogen to produce ammonia, consuming a few percent of the energy budget in the entire industry and making it the biggest consumer of hydrogen. The resulting ammonia is used extensively in fertilizer production.

Hydrogenation also converts unsaturated fats and oils to saturated fats and oils, with the major application being margarine production. Methanol is produced by hydrogenation of carbon dioxide using syngas. H₂ is used as a reducing agent for converting some ores to metals. Hydrogen can be used in fuel cells to produce electricity or burned to generate heat; when consumed in fuel cells, the only emission at the point of use is water vapor, while burning produces relatively little pollution but can form nitrogen oxides.

If produced with low or zero greenhouse gas emissions, hydrogen can decarbonize energy systems and provide intense heat for industrial production of steel, cement, glass, and chemicals, and serve as a low-carbon catalyst in steelmaking.