Hydrogen (H)

Hydrogen is a colourless, odourless gas and the simplest of all the chemical elements. Its most common isotope (see below) consists of just one proton and one electron. Although it is the lightest element it is by far the most abundant in the universe.

The diatomic molecule, formed under ordinary conditions from two atoms of hydrogen, will not react easily with other elements at room temperature due to the high level of energy required to break the bond between the atoms. Hydrogen will, however, react with most other elements at higher temperatures.

Essential Facts

Atomic number 1
Name Hydrogen
Symbol H
Atomic mass 1.008
Classification Other non-metal
State at 20°C Gas
Melting point -259.2°C
Boiling point -252.77°C


Hydrogen was discovered in 1766 by Henry Cavendish. He proved that hydrogen, or flammable air as it was then known, was distinct from other flammable gases by measuring its density.

When hydrogen burns it combines with oxygen to form water. This was confirmed by Cavendish in 1781 but had been observed by others before. It was this property of hydrogen that gave it its name.

In 1787, Antoine-Laurent Lavoisier coined the French word hydrogène for the element, from which the English hydrogen was derived soon after. The word was formed from the Greek stem hydr- (water) and -genḗs (something that produces) and so literally translates as “water-producer”.


There are two stable isotopes of hydrogen. The first is the standard form, the nucleus of which contains just a single proton. This form is sometimes known as protium and constitutes over 99.9% of the hydrogen mix.

The nucleus of heavy hydrogen, or deuterium (D), contains one proton and one neutron. Deuterium was first detected by Harold C. Urey in 1931 when the residue of a distillation of liquid hydrogen was analysed.

A third isotope, tritium (T), has a nucleus containing one proton and two neutrons. Tritium is radioactive, having a half-life of 12.5 years, and is formed naturally in the upper atmosphere. A reaction between cosmic rays and nitrogen produces oxygen, carbon and tritium. A reaction between neutrons and lithium in nuclear reactors also produces tritium, as well as helium.


There are several ways to produce hydrogen on an industrial scale, but the most important is a method called the catalytic steam-hydrocarbon process. This process uses a nickel catalyst at high temperature (650ºC-950ºC) to promote a reaction between high pressure steam and gaseous hydrocarbons. The results of the process are hydrogen and various carbon oxides.

Hydrogen can be produced on a smaller scale in a laboratory by various processes. These include the electrolysis of sodium hydroxide or a reaction between hydrochloric or sulphuric acid and zinc.


Most hydrogen is used in the production of ammonia following a reaction with nitrogen. Large quantities are also used in the production of methanol.

A compound of the second isotope, deuterium oxide (heavy water), is used as a moderator in nuclear reactors to slow the velocity of neutrons. When combusted with oxygen or fluorine, hydrogen can be used as rocket fuel.

Iron ore and some metal oxides can be reduced to the metal by a reaction with hydrogen. Margarine is produced by the hydrogenation of unsaturated vegetable oils and animal fats.

As mentioned above, hydrogen is the lightest of the elements, and so lighter than air. This property means it can be used to inflate balloons, such as weather balloons and airships. Rigid airships were used during the 1930s to transport thousands of passengers until the Hindenburg disaster of 1937. Most modern balloons use helium as, although it is twice as heavy as hydrogen, it is nonflammable.

Next Element

Helium (He)

Related Elements

Helium (He), Lithium (Li), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Sodium (Na), Sulphur / Sulfur (S), Chlorine (Cl), Iron (Fe), Nickel (Ni), Zinc (Zn)

Further Reading

24 February 1810: Death of Henry Cavendish, English Natural Philosopher

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