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Basic features
Electronic structure
Hydrogen is the lightest chemical element; its most common isotope comprises just one negatively charged electron, distributed around a positively charged proton (the nucleus of the hydrogen atom — all other atoms have more complex nuclei involving more protons or neutrons). The electron is bound to the proton by the Coulomb force, the electrical force that one stationary, electrically charged nanoparticle exerts on another. The hydrogen atom has special significance in quantum mechanics as a simple physical system for which there is an exact solution to the Schrödinger equation; from that equation, the experimentally observed frequencies and intensities of hydrogen's spectral lines can be calculated. Spectral lines are dark or bright lines in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies.

At standard temperature and pressure, hydrogen exists as the diatomic gas, H2, with a boiling point of 20.27 K, and a melting point of 14.02 K. Under extreme pressures, such as those at the center of gas giants, the molecules lose their identity and the hydrogen becomes a metal (metallic hydrogen). Under the extremely low pressure in space — virtually a vacuum — the element tends to exist as individual atoms, simply because it is statistically unlikely for them to combine. However, clouds of H2 and possibly single hydrogen atoms are said to form in H I and H II regions and are associated with star formation. Hydrogen plays a vital role in powering stars through the proton–proton and carbon–nitrogen cycle. These are nuclear fusion processes, which release huge amounts of energy in stars and other hot celestial bodies as hydrogen atoms combine into helium atoms.

At high temperatures, hydrogen gas can exist as a mixture of atoms, protons, and negatively charged hydride ions. This mixture has a high emissivity and absorptivity in the visible light range, and such emanations give rise to the light from the sun and other stars.

H2 is less soluble in water, alcohol, or ether than oxygen is. Its solubility and adsorption characteristics with various metals are very important in metallurgy (as many metals can suffer hydrogen embrittlement) and in developing safe ways to store it for use as a fuel.

Combustion
It reacts violently with chlorine and fluorine, forming hydrohalic acids, which can damage the lungs and other tissues. In air, it is highly flammable, burning at concentrations as low as 4% H2. When mixed with oxygen, hydrogen explodes upon ignition. A unique property of hydrogen is that its flame is nearly invisible in air. This makes it difficult to tell if a leak is burning, and carries the added risk that it is easy to walk into a hydrogen fire inadvertently.

See also: hydrogen atom.

Applications
Large quantities of H2 are needed in the petroleum and chemical industries. By far the largest application of H2 is for the processing ("upgrading") of fossil fuels. The key consumers of H2 in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocracking[1]. H2 has several other important uses.

used in the hydrogenation of fats and oils (found in items such as margarine), and in the production of methanol.
H2 is used in the manufacture of hydrochloric acid
H2 is used in certain welding methods
H2 is used in the reduction of metallic ores.
H2 is an ingredient in some rocket fuels.
H2 is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas.
Liquid H2 is used in cryogenic research, including superconductivity studies.
The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale.
Since H2 is 14.5 times lighter than air, it was once widely used as a lifting agent in balloons and airships. However, this use was curtailed after the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
Deuterium, an isotope of hydrogen (hydrogen-2), is used in nuclear fission applications as a moderator to slow neutrons, and 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 in the production of hydrogen bombs, as an isotopic label in the biosciences, and as a radiation source in luminous paints.
Hydrogen as an energy source
Hydrogen is not a pre-existing source of energy like fossil fuels, but a carrier, much like a battery. There are no "hydrogen wells" or "hydrogen mines" on Earth, so H2 cannot be considered a primary energy source such as fossil fuels or uranium. Since H2 is so light, any amount present on earth will float up into the atmosphere and out into space. H2 can however be burned in internal combustion engines, an approach advocated by BMW's experimental hydrogen car. There are several methods of storing hydrogen for transport applications, the most commonly-used being gaseous storage in gas cylinders similar to those used for the storage of any pressurised gas. Alternatives include storage as metal or chemical hydrides, cryogenic storage of liquid hydrogen (as in BMW's hydrogen internal combustion engine car) and research points to nanomaterials that will be able to store hydrogen more efficiently than any of the methods above. Crucially, there is another option for those wishing to drive a hydrogen-fuelled vehicle: on-board reformation. Often over-looked, this concept involves refuelling your vehicle with a hydrocarbon fuel such as ethanol, methanol, gasoline, diesel (preferably the biofuel alternatives) and then using an on-board reformer (a device that strips hydrogen from hydrocarbon fuel to leave carbon dioxide and water vapour as waste products) to convert the fuel to hydrogen just before it is needed by the engine.