Hydrogen

Hydrogen is the chemical element with atomic number 1, the lightest and simplest of all the elements. Its neutral atom contains one proton and one electron. Under ordinary conditions it exists as a colorless, odorless, tasteless gas made of two-atom molecules written H2. Hydrogen is the most plentiful element in the universe, and it powers the Sun and other stars through nuclear fusion. On Earth it is found mostly combined with other elements, most familiarly with oxygen in water.

History of hydrogen

Hydrogen gas was being made in laboratories long before anyone understood what it was. The Jefferson Lab element profile records that Robert Boyle produced the gas by 1671 while experimenting with iron and acids. Henry Cavendish first recognized it as a distinct substance in 1766. The name comes from Greek words meaning “water forming,” a reference to the water that results when the gas burns.

Its heavier forms were identified in the twentieth century. Deuterium, a stable variety, was discovered in 1932, and tritium followed in 1934. Scientists also came to see hydrogen’s central role in astronomy. The Royal Society of Chemistry notes that the Sun’s energy comes from hydrogen nuclei fusing into helium, which makes the element the fuel of the stars as well as a building block of chemistry on Earth.

Properties of hydrogen

Hydrogen has an atomic mass of about 1.008, a figure that reflects the mixture of forms found in nature. It is classed as a nonmetal. At room temperature it is a gas with an extremely low density of about 0.00008988 grams per cubic centimeter, which makes it the lightest gas known.

Its phase changes happen at very low temperatures. According to Jefferson Lab, hydrogen melts at 13.81 kelvin (minus 259.34 degrees Celsius) and boils at 20.28 kelvin (minus 252.87 degrees Celsius). Storing it as a liquid therefore requires extreme cooling, which is one reason handling hydrogen is a demanding engineering task.

Abundance depends on where you look. Jefferson Lab estimates that hydrogen makes up about 90 percent of the visible universe. Its listed abundance is about 1,400 milligrams per kilogram of Earth’s crust and about 108,000 milligrams per liter of ocean water, where it is mostly locked inside water molecules.

Chemically, hydrogen reacts readily with oxygen, releasing energy and forming water. That reaction explains why hydrogen works as a fuel, and why energy must be spent to pull the element out of water or other compounds. Because each atom has only one electron, hydrogen can form bonds with many other elements. It is a component of water, acids, hydrocarbons and living tissue.

Isotopes of hydrogen

An isotope is a form of an element whose atoms contain the same number of protons but a different number of neutrons. Hydrogen has three common isotopes, and each has its own name.

Protium is the ordinary form, with one proton and no neutrons. It accounts for nearly all natural hydrogen. Deuterium has one neutron and is stable. Water made with deuterium is known as heavy water, which is used in research and in some nuclear reactors. Tritium has two neutrons and is radioactive. Jefferson Lab lists two stable isotopes in total, protium and deuterium, so tritium is the only one of the three that decays.

How hydrogen is produced

Because hydrogen is bound up in other compounds, it must be separated before it can be used. The U.S. Energy Information Administration describes two main routes.

The first is steam-methane reforming. High-temperature steam reacts under pressure with methane, the main component of natural gas, in the presence of a catalyst. The reaction yields hydrogen, carbon monoxide and a small amount of carbon dioxide. The agency states that this method supplies nearly all commercially produced hydrogen in the United States.

The second is electrolysis, in which an electric current splits water into hydrogen and oxygen. The process itself creates no other emissions, but its overall footprint depends on the power source. Electricity on the grid can come from renewable energy, nuclear plants or fossil fuels. Other pathways still under research include thermochemical conversion of biomass, solar-driven splitting of water, and biological production using bacteria and microalgae.

Applications and examples of hydrogen

The U.S. Department of Energy reports that the United States produces about 10 million metric tons of hydrogen a year, mostly for oil refining and fertilizer production. Fertilizer manufacturing depends on the Haber process, which combines hydrogen with nitrogen to make ammonia. Hydrogen is also used to harden fats and oils, such as peanut oil, through a reaction called hydrogenation.

Other uses are more specialized. Combined with liquid oxygen, hydrogen serves as rocket fuel. Liquid hydrogen is used in laboratory studies of superconductors, materials that carry electricity without resistance at very low temperatures. In stars, hydrogen fusion is the energy source, and researchers continue to study fusion as a possible power source on Earth.

Hydrogen in energy

The Department of Energy notes that hydrogen carries about three times as much energy as gasoline by weight. Its very low density as a gas means it must be compressed or cooled to store a useful amount, but the high energy per unit of mass makes it attractive where weight matters.

Fuel cells convert hydrogen into electricity, and the department lists applications such as cell phone towers, data centers, hospitals and supermarkets. It also names trucks, trains, ships and port equipment as targets for hydrogen fuel. Hydrogen can serve as a form of long-term energy storage and may help improve the efficiency of the electric grid. In addition, it is proposed for industries that are hard to decarbonize, including steel, cement and ammonia.

The climate benefit depends on how the hydrogen is made. Most of today’s hydrogen comes from natural gas, so a shift toward electrolysis powered by low-carbon electricity would change the picture. The cost and scale of that shift are still being worked out.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue