Hydrogen

The first recorded instance of hydrogen made by human action was in the first half of the 1500s. Theophrastus Paracelsus, a physician, dissolved iron in sulfuric acid and observed the release of a gas. He is reported to have said of the experiment, "Air arises and breaks forth like a wind." He did not, however, discover any of hydrogen's properties.

Turquet De Mayerne repeated Paracelsus's experiment in 1650 and found that the gas was flammable. Neither Paracelsus nor De Mayerne proposed that hydrogen could be a new element. Indeed, Paracelsus believed there were only three elements - the tria prima - salt, sulfur, and mercury - and that all other substances were made of different combinations of these three.

In 1670 Robert Boyle added iron to sulfuric acid. He showed the resulting gas only burnt if air was present and that a fraction of the air (we would now call it oxygen) was consumed by the burning. He also found that the combustion products were heavier than the starting materials.

Hydrogen was first recognized as a distinct element by Henry Cavendish in 1766, when he prepared it by reacting hydrochloric acid with zinc. He described hydrogen as "inflammable air from metals" and established that it was the same material (by its reactions and its density) regardless of which metal and which acid he had used to produce it. Cavendish also observed that when the substance was burned, it produced water.

Lavoisier later named the element hydrogen (1783). The name comes from the Greek 'hydro' meaning water and 'genes' meaning forming - hydrogen is one of the two water forming elements.

In 1806, with hydrogen well-established as an element, Humphry Davy pushed a strong electric current through pure water. He found hydrogen and oxygen were formed. The experiment demonstrated that electricity could pull substances apart into their constituent elements. Davy realized that substances were bound together by an electrical phenomenon; he had discovered the true nature of chemical bonding.

Hydrogen is highly flammable and has an almost invisible flame, which can lead to accidental burns.

Hydrogen is the simplest element of all, and the lightest. It is also by far the most common element in the Universe. Over 90 percent of the atoms in the Universe are hydrogen.

In its commonest form, the hydrogen atom is made of one proton, one electron, and no neutrons. Hydrogen is the only element that can exist without neutrons.

Hydrogen is a colorless, odorless gas which exists, at standard temperature and pressure, as diatomic molecules, H₂.

It burns and forms explosive mixtures in air and it reacts violently with oxidants.

On Earth, the major location of hydrogen is in water, H₂O. There is little free hydrogen on Earth because it is so light it escapes from the atmosphere into space.

Large quantities of hydrogen are used in the Haber process (production of ammonia), hydrogenation of fats and oils, methanol production, hydrocracking, and hydrodesulfurization.

Hydrogen is also used in metal refining. Liquid hydrogen is used as a rocket fuel, for example powering the Space Shuttle's lift-off and ascent into orbit. Liquid hydrogen and oxygen are held in the Shuttle's large, external fuel tank. (See image left.) Hydrogen's two heavier isotopes (deuterium and tritium) are used in nuclear fusion. The hydrogen economy has been proposed as a replacement for our current hydrocarbon (oil and coal based) economy. The basis of the hydrogen economy is that energy is produced when hydrogen combusts with oxygen and the only by-product from the reaction is water. At the moment, however, the hydrogen for hydrogen-powered cars is produced from hydrocarbons. Only when solar or wind energies, for example, can be used commercially to split water into hydrogen and oxygen will a true hydrogen economy be possible.

Hydrogen is prepared commercially by reacting superheated steam with methane or carbon. In the laboratory, hydrogen can be produced by the action of acids on metals such as zinc or magnesium, or by the electrolysis of water (shown on the left).

Hydrogen has three isotopes, ¹H (protium), ²H (deuterium) and ³H (tritium). Its two heavier isotopes (deuterium and tritium) are used for nuclear fusion. Protium is the most abundant isotope, and tritium the least abundant. Tritium is unstable with a half-life of about 12 years 4 months.