hydrogen

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• U.S. Energy Information Administration - Hydrogen explained
• Chemicool - Hydrogen Element Facts
• National Center for Biotechnology Information - PubChem - Hydrogen
• Royal Society of Chemistry - Hydrogen
• Chemistry LibreTexts - Hydrogen

• hydrogen - Children's Encyclopedia (Ages 8-11)
• hydrogen - Student Encyclopedia (Ages 11 and up)

While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.

Our editors will review what you’ve submitted and determine whether to revise the article.

• U.S. Energy Information Administration - Hydrogen explained
• Chemicool - Hydrogen Element Facts
• National Center for Biotechnology Information - PubChem - Hydrogen
• Royal Society of Chemistry - Hydrogen
• Chemistry LibreTexts - Hydrogen

• hydrogen - Children's Encyclopedia (Ages 8-11)
• hydrogen - Student Encyclopedia (Ages 11 and up)

Emeritus Professor of Chemistry, University of California, Berkeley. Author of The Synthesis and Characterization of Inorganic Compounds and others.

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hydrogen (H), a colourless, odourless, tasteless, flammable gaseous substance that is the simplest member of the family of chemical elements. The hydrogen atom has a nucleus consisting of a proton bearing one unit of positive electrical charge; an electron, bearing one unit of negative electrical charge, is also associated with this nucleus. Under ordinary conditions, hydrogen gas is a loose aggregation of hydrogen molecules, each consisting of a pair of atoms, a diatomic molecule, H₂. The earliest known important chemical property of hydrogen is that it burns with oxygen to form water, H₂O; indeed, the name hydrogen is derived from Greek words meaning “maker of water.”

Although hydrogen is the most abundant element in the universe (three times as abundant as helium, the next most widely occurring element), it makes up only about 0.14 percent of Earth’s crust by weight. It occurs, however, in vast quantities as part of the water in oceans, ice packs, rivers, lakes, and the atmosphere. As part of innumerable carbon compounds, hydrogen is present in all animal and vegetable tissue and in petroleum. Even though it is often said that there are more known compounds of carbon than of any other element, the fact is that, since hydrogen is contained in almost all carbon compounds and also forms a multitude of compounds with all other elements (except some of the noble gases), it is possible that hydrogen compounds are more numerous.

Elementary hydrogen finds its principal industrial application in the manufacture of ammonia (a compound of hydrogen and nitrogen, NH₃) and in the hydrogenation of carbon monoxide and organic compounds.

Hydrogen has three known isotopes. The mass numbers of hydrogen’s isotopes are 1, 2, and 3, the most abundant being the mass 1 isotope generally called hydrogen (symbol H, or 1 H) but also known as protium. The mass 2 isotope, which has a nucleus of one proton and one neutron and has been named deuterium, or heavy hydrogen (symbol D, or 2 H), constitutes 0.0156 percent of the ordinary mixture of hydrogen. Tritium (symbol T, or 3 H), with one proton and two neutrons in each nucleus, is the mass 3 isotope and constitutes about 10 −15 to 10 −16 percent of hydrogen. The practice of giving distinct names to the hydrogen isotopes is justified by the fact that there are significant differences in their properties.

Paracelsus, physician and alchemist, in the 16th century unknowingly experimented with hydrogen when he found that a flammable gas was evolved when a metal was dissolved in acid. The gas, however, was confused with other flammable gases, such as hydrocarbons and carbon monoxide. In 1766 Henry Cavendish, English chemist and physicist, showed that hydrogen, then called flammable air, phlogiston, or the flammable principle, was distinct from other combustible gases because of its density and the amount of it that evolved from a given amount of acid and metal. In 1781 Cavendish confirmed previous observations that water was formed when hydrogen was burned, and Antoine-Laurent Lavoisier, the father of modern chemistry, coined the French word hydrogène from which the English form is derived. In 1929 Karl Friedrich Bonhoeffer, a German physical chemist, and Paul Harteck, an Austrian chemist, on the basis of earlier theoretical work, showed that ordinary hydrogen is a mixture of two kinds of molecules, ortho-hydrogen and para-hydrogen. Because of the simple structure of hydrogen, its properties can be theoretically calculated relatively easily. Hence hydrogen is often used as a theoretical model for more complex atoms, and the results are applied qualitatively to other atoms.

Physical and chemical properties

The Table lists the important properties of molecular hydrogen, H₂. The extremely low melting and boiling points result from weak forces of attraction between the molecules. The existence of these weak intermolecular forces is also revealed by the fact that, when hydrogen gas expands from high to low pressure at room temperature, its temperature rises, whereas the temperature of most other gases falls. According to thermodynamic principles, this implies that repulsive forces exceed attractive forces between hydrogen molecules at room temperature—otherwise, the expansion would cool the hydrogen. In fact, at −68.6° C attractive forces predominate, and hydrogen, therefore, cools upon being allowed to expand below that temperature. The cooling effect becomes so pronounced at temperatures below that of liquid nitrogen (−196° C) that the effect is utilized to achieve the liquefaction temperature of hydrogen gas itself.

Hydrogen is transparent to visible light, to infrared light, and to ultraviolet light to wavelengths below 1800 Å. Because its molecular weight is lower than that of any other gas, its molecules have a velocity higher than those of any other gas at a given temperature and it diffuses faster than any other gas. Consequently, kinetic energy is distributed faster through hydrogen than through any other gas; it has, for example, the greatest heat conductivity.

A molecule of hydrogen is the simplest possible molecule. It consists of two protons and two electrons held together by electrostatic forces. Like atomic hydrogen, the assemblage can exist in a number of energy levels.

Ortho-hydrogen and para-hydrogen

Two types of molecular hydrogen (ortho and para) are known. These differ in the magnetic interactions of the protons due to the spinning motions of the protons. In ortho-hydrogen, the spins of both protons are aligned in the same direction—that is, they are parallel. In para-hydrogen, the spins are aligned in opposite directions and are therefore antiparallel. The relationship of spin alignments determines the magnetic properties of the atoms. Normally, transformations of one type into the other (i.e., conversions between ortho and para molecules) do not occur and ortho-hydrogen and para-hydrogen can be regarded as two distinct modifications of hydrogen. The two forms may, however, interconvert under certain conditions. Equilibrium between the two forms can be established in several ways. One of these is by the introduction of catalysts (such as activated charcoal or various paramagnetic substances); another method is to apply an electrical discharge to the gas or to heat it to a high temperature.

The concentration of para-hydrogen in a mixture that has achieved equilibrium between the two forms depends on the temperature as shown by the following figures:

Essentially pure para-hydrogen can be produced by bringing the mixture into contact with charcoal at the temperature of liquid hydrogen; this converts all the ortho-hydrogen into para-hydrogen. The ortho-hydrogen, on the other hand, cannot be prepared directly from the mixture because the concentration of para-hydrogen is never less than 25 percent.

The two forms of hydrogen have slightly different physical properties. The melting point of para-hydrogen is 0.10° lower than that of a 3:1 mixture of ortho-hydrogen and para-hydrogen. At −252.77° C the pressure exerted by the vapour over liquid para-hydrogen is 1.035 atmospheres (one atmosphere is the pressure of the atmosphere at sea level under standard conditions, equal to about 14.69 pounds per square inch), compared with 1.000 atmosphere for the vapour pressure of the 3:1 ortho–para mixture. As a result of the different vapour pressures of para-hydrogen and ortho-hydrogen, these forms of hydrogen can be separated by low-temperature gas chromatography, an analytical process that separates different atomic and molecular species on the basis of their differing volatilities.