Hydrogen is the lightest element on the Periodic Table. At room temperature, it is a transparent, highly flammable gas. Because hydrogen is so light, all atmospheric hydrogen has already evaporated out into space, although it can be created artificially. In industrial settings, hydrogen is usually synthesized by exposing a metal such as zinc to a strong acid. The result is a diatomic gas, H₂.

Hydrogen is used in massive quantities in the petroleum and chemical industries. In a petrochemical plant, hydrogen is used for hydrodealkylation, hydrodesulfurization, and hydrocracking, all methods of refining crude oil for wider use. In the food industry, hydrogen is used to hydrogenate oils or fats, which permits the production of margarine from liquid vegetable oil. Hydrogen is used to produce methanol and hydrochloric acid, as well as being used as a reducing agent for metal ores.

In the early 20th century, hydrogen was used as a lifting gas for airships, although this ended for good in 1937, when the Hindenburg disaster was (erroneously) blamed on the fuel. Today, helium or heated gas is used for

airships instead.

Hydrogen also has applications in physics and engineering. Hydrogen is used as a shielding gas for welding, isolating the site of the weld from atmospheric gases such as oxygen and nitrogen. It is used for cooling rotors in electrical power generators because of its high thermal conductivity. Liquid hydrogen is colder than 14 Kelvin, and is used for research in cryogenics and superconductivity.

Hydrogen's isotopes, especially deuterium, are used in nuclear reactors. Deuterium can be used as a neutron moderator for fission reactions, or a fuel for fusion reactions. Tritium, another isotope of hydrogen, is used as a radiation source in luminous paints and is a component of hydrogen bombs.

More recently, using hydrogen as a clean fuel has been discussed. However, the environmentally friendly reputation of hydrogen fuel cells is unjustified, as large quantities of fossil fuels are consumed to generate the hydrogen itself.