Hydrogen

• 1. Introduction

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  Hydrogen carries the symbol H and the atomic number 1, the very first entry in the catalogue of the elements. It is the lightest of them all, and the most abundant in the universe, making up about 75% of all normal matter. Stars, including the Sun, are mostly hydrogen, held in a hot plasma state. Yet for all its cosmic dominance, no one recognized it as a distinct substance until the 1760s. Henry Cavendish, working between 1766 and 1781, identified the gas and noticed something strange. When it burned, it produced water. That single observation gave the element its name, built from the Greek words for water and to generate. From there, hydrogen would become a thread running through some of the deepest questions in science. How did the universe cool enough to make the first atoms? Why did a single proton and a single electron unlock the theory of atomic structure? And why does a gas this useful remain so difficult and dangerous to store?

• 2. A Proton And An Electron

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  The most common isotope of hydrogen consists of one proton, one electron, and no neutrons. That stark simplicity made the hydrogen atom central to the development of the theory of atomic structure. The ground state energy level of its electron is minus 13.6 electronvolts, equal to an ultraviolet photon of roughly 91 nanometers wavelength. Understanding the colors of light that hydrogen absorbs and emits became a crucial part of the development of quantum mechanics.

  The hydrogen spectral series corresponds to light emitted as the electron drops from higher to lower energy levels. Early quantum theory pictured the electron orbiting the proton much as Earth orbits the Sun, except that electrostatic attraction holds them together rather than gravity. Because angular momentum was postulated to be discrete, the electron could occupy only certain allowed distances and certain allowed energies.

  Hydrogen is the only neutral atom for which the equation can be directly solved, a fact that significantly contributed to the understanding of quantum mechanics. Each energy level is further split by spin interactions between the electron and proton into four hyperfine levels. High-precision values for these levels are required for definitions of physical constants, and quantum calculations have identified nine separate contributions to them. The largest contribution is the eigenvalue from the Dirac equation, with smaller terms for relativistic recoil, the self-energy, and vacuum polarization.

• 3. Three Named Twins

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  Hydrogen is unique among the elements in that its isotopes are given their own distinct names. The most common, protium, has an abundance above 99.98 percent and is the only stable isotope with no neutrons. Its rarely-used formal name reflects a nucleus of a single proton.

  Deuterium, the other stable isotope, carries one proton and one neutron. Nearly all of its nuclei in the universe are thought to have been produced in Big Bang nucleosynthesis and to have endured since. It is not radioactive and poses no significant toxicity hazard. Water enriched with deuterium is called heavy water, used as a neutron moderator and coolant in nuclear reactors, and deuterium is a potential fuel for commercial nuclear fusion. Harold Urey discovered it in December 1931, and his group discovered heavy water the following year.

  Tritium, with one proton and two neutrons, is radioactive. It decays into helium-3 through beta decay with a half-life of 12.32 years. That radioactivity makes it useful in luminous paint, lighting the hands and dial-markers of watches, with the watch glass keeping the small amount of radiation inside the case. Ernest Rutherford, Mark Oliphant, and Paul Harteck prepared it in 1934. Cosmic rays striking atmospheric gases produce small natural amounts, and nuclear weapons tests have released more.

• 4. The Shape Of The Molecule

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  Under standard conditions, hydrogen is a gas of diatomic molecules officially called dihydrogen, a colorless, odorless, flammable gas. Its bond is remarkably strong, with a bond dissociation energy of 435.7, which is the thermodynamic reason the molecule is relatively unreactive.

  Molecular hydrogen also exists as two nuclear isomers that differ in the spin states of their nuclei. In the ortho form the nuclear spins are parallel; in the para form they are opposed. At room temperature or warmer, equilibrium hydrogen gas is about 25% para and 75% ortho. The ortho form is an excited state, higher in energy than the para form by 1.455, and when cooled to low temperature it converts to para over several minutes.

  This quirk has stark practical consequences. James Clerk Maxwell once noticed that the specific heat capacity of hydrogen departs from that of a diatomic gas below room temperature and begins to resemble a monatomic gas at cryogenic temperatures. Quantum theory later explained this through the widely-spaced rotational energy levels caused by hydrogen's low mass. The same ortho-to-para conversion matters in liquefaction, because it is exothermic and can release enough heat to evaporate most of the liquid. Catalysts such as ferric oxide and activated carbon are used during cooling to prevent that loss.

• 5. From Gas To Solid

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  Liquid hydrogen can exist below hydrogen's critical point of 33 K, but to be fully liquid at atmospheric pressure it must be cooled to 20.28 K. James Dewar first liquefied hydrogen in 1898 using regenerative cooling and his own invention, the vacuum flask. He produced solid hydrogen the next year.

  Solid hydrogen forms below the melting point of 14.01 K, and it comes in distinct solid phases known as Phase I through Phase V, each with a characteristic molecular arrangement. At the triple point, liquid and solid can coexist in a mixture known as slush hydrogen.

  Metallic hydrogen sits at the far extreme. Obtained only at pressures in excess of 400 e9Pa, it is an electrical conductor and is believed to exist deep within giant planets like Jupiter. When ionized, hydrogen becomes a plasma, the form in which it exists within stars.

• 6. The First Atoms

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  About 370,000 years after the Big Bang, neutral hydrogen atoms formed during the recombination epoch, when the universe had expanded and the plasma had cooled enough for electrons to remain bound to protons. Protons themselves had appeared in the first second. Hydrogen makes up 75% of normal matter by mass and more than 90% by number of atoms.

  In the interstellar medium, neutral hydrogen is called H I and ionized hydrogen is called H II. Radiation from stars ionizes H I into H II, carving out spheres of ionized gas around them. Neutral hydrogen dominated the universe until the birth of stars during the era of reionization, when bubbles of ionized hydrogen grew and merged over hundreds of millions of years.

  These clouds are the source of the 21-centimeter hydrogen line, at 1420, detected to probe primordial hydrogen. Molecular clouds of hydrogen are tied to star formation, and hydrogen powers stars through the proton-proton reaction in lower-mass stars and the CNO cycle in stars more massive than the Sun. The trihydrogen cation, generated when cosmic rays ionize molecular hydrogen, is one of the most abundant ions in the universe and has even been observed in the upper atmosphere of Jupiter.

• 7. Lift And Disaster

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  Because hydrogen has only 7% the density of air, it was once widely used as a lifting gas in balloons and airships. By 1806 it was already being used to fill balloons. A German count promoted the idea of rigid airships lifted by hydrogen, the first of which had its maiden flight in 1900. Regularly-scheduled flights began in 1910, and by the outbreak of World War I in August 1914 they had carried 35,000 passengers without a serious incident.

  The British airship R34 made the first non-stop transatlantic crossing in 1919, and regular passenger service resumed in the 1920s. During World War II, hydrogen-lifted blimps served as observation platforms and bombers, especially on the US Eastern seaboard.

  Then came the Hindenburg, which caught fire over New Jersey on the 6th of May 1937. The hydrogen filling the airship ignited, possibly by static electricity, and burst into flames. Commercial hydrogen airship travel ceased after the disaster. Hydrogen is still chosen over the non-flammable but more expensive helium as a lifting gas for weather balloons.

• 8. Making And Keeping It

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  Nearly all of the world's hydrogen comes from fossil fuels, with less than 1% produced by low-emissions technologies in 2025. The dominant method is steam methane reforming, in which steam reacts with methane at temperatures between 1000 and 1400 K. Producing one tonne of hydrogen this way emits 6.6 tonnes of carbon dioxide, before counting vented and fugitive methane from the gas supply itself.

  Electrolysis of water offers a cleaner route. Commercial electrolyzers use nickel-based catalysts in strongly alkaline solution, and hydrogen made this way with renewable energy is called green hydrogen. The process remains more expensive than producing hydrogen from methane without carbon capture. Pyrolysis of methane offers another path, yielding hydrogen gas and solid carbon that can be sold or landfilled.

  Keeping hydrogen is its own challenge. It dissolves only poorly in solvents, and liquefaction is impractical given its low critical temperature. Hydrogen carriers that reversibly bind the gas have drawn much attention, with the key measure being the weight percent of hydrogen they hold. Ammonia borane holds 19.8 weight percent, but cannot re-add hydrogen once released, making it an irreversible carrier. The gas's high solubility in metals also causes embrittlement, complicating the design of pipelines and storage tanks. That fragility, more than any chemical limit, shapes how hydrogen could one day power ships, aircraft, and the Space Shuttle main engines, where liquid hydrogen and liquid oxygen serve as cryogenic propellants.