Hydrogen () is a chemical element in the periodic table that has the symbol H and atomic number 1. A colorless, odorless, non-metal, univalent, highly flammable diatomic gas, hydrogen is the lightest and most abundant element in the universe and is present in water and in all organic compounds and living organisms. Hydrogen is able to react chemically with most elements. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. This element is used in ammonia production, as a lifting gas, an alternative fuel, and more recently as a power-source of fuel cells.

In laboratory, it is prepared by reaction of acids on metals like zinc. For production in large scale, electrolysis of water is a widely used method. Scientists are now trying to develop new methods that involve use of green algae for hydrogen production.

Notable characteristics

Hydrogen was the lightest chemical element with its most common isotope consisting of just a single proton and electron. At standard temperature and pressure conditions, hydrogen forms a diatomic gas, H₂, with a boiling point of only 20.27 K and a melting point of 14.02 K. Under exceedingly high pressures, like those found at the center of gas giants, the molecules lose their identity and the hydrogen becomes a liquid metal (see metallic hydrogen). Under the exceedingly low pressure conditions found in space, hydrogen tends to exist as individual atoms, simply because there is no way for them to combine; clouds of H₂ form and are associated with star formation.

This element plays a vital role in powering the universe through the proton-proton reaction and carbon-nitrogen cycle (these are nuclear fusion processes that release huge amounts of energy through combining two hydrogen atoms into one helium).

Applications

Large quantities of hydrogen are needed industrially, notably in the Haber process for the production of ammonia, the hydrogenation of fats and oils, and the production of methanol. Other uses that require hydrogen:

hydrodealkylation, hydrodesulfurization, and hydrocracking.
manufacture of hydrochloric acid, welding, rocket fuels, and the reduction of metallic ores.
liquid hydrogen is used in cryogenic research including superconductivity studies,
tritium is produced in nuclear reactors and is used in hydrogen bomb construction.
It is fourteen and a half times lighter than air and at one time was widely used as a lifting agent in balloons and zeppelins until the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
Deuterium is used in nuclear applications as a moderator to slow down neutrons, and deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
Tritium is used as an isotopic label in the biosciences, as a radiation source in luminous paints.

Hydrogen can be burned in internal combustion engines, and a fleet of hydrogen burning cars is maintained by Chrysler-BMW. Hydrogen fuel cells are being looked into as a way to provide potentially cheap, pollution-free power.

History

Hydrogen (French for water-maker, from Greek hudôr, "water" and gennen, "generate") was first recognized as a distinct substance in 1776 by Henry Cavendish. Antoine Lavoisier gave the element its name.

Occurrence

Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is found in great abundance stars and gas giant planets. Relative to its great abundance elsewhere, hydrogen is very rare in the earth's atmosphere (1 ppm by volume). The most common source for this element on earth is water which is composed two parts hydrogen to one part oxygen (H₂O). Other sources are; most forms of organic matter which includes all known life forms, coal, fossil fuels and natural gas. Methane (CH₄), which is a byproduct of organic decay, is an increasingly important source of hydrogen.

Hydrogen is prepared in several different ways; steam on heated carbon, hydrocarbon decomposition with heat, action of sodium or potassium hydroxide on aluminum, water electrolysis, or by displacement from acids with certain metals.

Commercial bulk hydrogen is usually manufactured by decomposing natural gas.

Compounds

The lightest of all gases, hydrogen combines with most other elements to form compounds. Hydrogen has an electronegativity of 2.2, so it forms compounds where it is the more non-metallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H^- ions or just as a solute within the other element (as in Palladium hydride). The latter tend to be covalent, since the H⁺ ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like H₃O⁺ as the protons latch on to something.

Hydrogen combines with oxygen to form water, H₂O, and releases a lot of energy in doing so, burning explosively in air. Deuterium oxide, or D₂O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and the study of the properties of these compounds is called organic chemistry.

Forms

Under normal conditions hydrogen gas is a mix of two different kinds of molecules which differ from one another by the relative spin of the nuclei. These two forms are known as ortho- and para-hydrogen (this is different than isotopes, see below). In ortho-hydrogen the nuclear spins are parallel (form a triplet), while in para they are antiparallel (form a singlet). At standard conditions hydrogen is comprised of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depend on temperature but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. In low temperatures (around boiling point), the equilibrium state is comprised of almost only para form.

The conversion process between the forms is slow and if hydrogen is cooled down and condensed rapidly, it contains large quantities of the ortho form. It is important in preparation and storage of liquid hydrogen since the ortho-para conversion produces more heat than the heat of its evaporation and a lot of hydrogen can be lost by evaporation in this way during several days after liquifying. Therefore, some catalysts of the ortho-para convertion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.

Isotopes

The most common hydrogen isotope, protium, has no neutrons, although there are two others - deuterium with one, and radioactive tritium with two neutrons. The two stable isotopes are protium (H-1) and deuterium (H-2, D). Deuterium comprises 0.0184-0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. A radioactive isotope, tritium (T or H-3) has one proton and two neutrons.

Hydrogen is the only element that has different names for its isotopes.

Precautions

Hydrogen is a highly flammable gas. It also reacts violently with chlorine and fluorine. D₂O, or heavy water, is toxic to many species. The quantity required to kill a human, however, is substantial.

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hydrogen".

Hydrogen