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Hydrogen (Latin: hydrogenium, from Greek: hydro: water, genes: forming) is the chemical element in the periodic table that has the symbol H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, non-metallic, univalent, highly flammable diatomic gas. Hydrogen is the lightest and most abundant element in the universe. It is present in water and in all organic compounds and living organisms. Hydrogen is able to react chemically with most other 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, as an alternative fuel, and more recently as a power-source of fuel cells.

In the laboratory, hydrogen is prepared by reaction of acids on metals such as zinc. For production in large scale commercial bulk hydrogen is usually manufactured by decomposing natural gas. Electrolysis of water is a simple although inefficient method. Scientists are now researching new methods for hydrogen production. One of them involves use of green algae. Another promising method involves the conversion of biomass derivatives such as glucose or sorbitol, which can be done at low temperatures through the use of a new catalyst.

Notable characteristics
Hydrogen is 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 atom.)

The hydrogen atom
Main article: Hydrogen atom.

A hydrogen atom is an atom of the element hydrogen. It is composed of a single negatively charged electron, distributed around the positively charged proton which is the nucleus of the hydrogen atom. The electron is bound to the proton by the Coulomb force.

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. Hydrogen is used in hydrodealkylation, hydrodesulfurization, and hydrocracking. Other uses:


 * The element is used in the manufacture of hydrochloric acid, welding, and the reduction of metallic ores.
 * It is used in rocket fuels.
 * Liquid hydrogen is used to perform cryogenic research, including superconductivity studies.
 * Since hydrogen is fourteen and a half times lighter than air, it was once widely used as a lifting agent in balloons and airships. However this use was curtailed when the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
 * Deuterium, an isotope (hydrogen-2) of hydrogen, is used in nuclear fission applications as a moderator to slow down neutrons, and is also used in nuclear fusion reactions. Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
 * Tritium (hydrogen-3), produced in nuclear reactors, is used to construct hydrogen bombs. It is also used as an isotopic label in the biosciences and 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 (see Hydrogen car). Hydrogen fuel cells are being looked into as a way to provide power with lower emissions than hydrogen burning internal combustion engines. The low emissions of hydrogen in internal combustion engines and fuel cells is offset by the pollution created by hydrogen production.

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. Cavendish stumbled upon it when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a compound of mercury (and not of the acid), he was still able to describe many of hydrogen's properties quite accurately. Antoine Lavoisier gave the element its name and proved that water was made of hydrogen and oxygen. One of its first uses was for balloons. The hydrogen was obtained by mixing sulfuric acid and iron. Deuterium, one isotope of hydrogen, was discovered by Harold C. Urey by distilling a sample of water multiple times. Urey received a Nobel prize for his discovery in 1934. In the same year, the third isotope, tritium, was discovered.

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 in 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 include most forms of organic matter (currently all known life forms), coal, fossil fuels and natural gas. Methane (CH4) is an increasingly important source of hydrogen.

Hydrogen can be prepared in several different ways: steam on heated carbon, hydrocarbon decomposition with heat, reaction of a strong base in an aqueous solution with aluminium, water electrolysis, or displacement from acids with certain metals.

Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700-1100 &deg;C), steam reacts with methane to yield carbon monoxide and hydrogen.


 * CH4 + H2O &rarr; CO + 3 H2

Additional hydrogen can be recovered from the carbon monoxide through the water-gas shift reaction:


 * CO + H2O &rarr; CO2 + H2

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 hydronium (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 from 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 composed 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 conversion 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
Hydrogen is the only element (other than radon) that has different names for its isotopes. The symbols D and T (instead of 2H and 3H) are sometimes used for deuterium and tritium, although this is not officially sanctioned. (The symbol P is already in use for phosphorus and is not available for protium.) The most common isotope of hydrogen, this stable isotope has a nucleus consisting of a single proton; hence the descriptive, although rarely used, name protium. The other stable isotope is deuterium, with an extra neutron in the nucleus. Deuterium comprises 0.0184-0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. The third naturally-occurring hydrogen isotope is the radioactive tritium. The tritium nucleus contains two neutrons in addition to the proton. It decays through beta decay and has a half-life of 12.32 years. Hydrogen-4 was synthesised by bombarding tritium with fast-moving deuterium nuclei. It decays through neutron emission and has a half-life of 9.93696x10-23 seconds. In 2001 scientists detected hydrogen-5 by bombarding a hydrogen target with heavy ions. It decays through neutron emission and has a half-life of 8.01930x10-23 seconds. Hydrogen-6 decays through triple neutron emission and has a half-life of 3.26500-22 seconds. In 2003 hydrogen-7 was created (article) at the RIKEN laboratory in Japan by colliding a high-energy beam of helium-8 atoms with a cryogenic hydrogen target and detecting tritons&mdash;the nuclei of tritium atoms&mdash;and neutrons from the break up of Hydrogen-7, the same method used to produce and detect Hydrogen-5.
 * 1H
 * 2H
 * 3H
 * 4H
 * 5H
 * 6H
 * 7H

Precautions
Hydrogen is a highly flammable gas burning at concentrations as low as 4%. It also reacts violently with chlorine and fluorine, forming hydrohalic acids that can cause damage to the lungs and other biological tissues. When mixed with oxygen, hydrogen explodes on ignition.