HYDROGEN, H (Latin: hydrogenium; EN: hydrogen; DE: Wasserstoff; FR: hydrogene; ES: hidrogeno; RU: водород), is the chemical element of the periodic table of Mendeleev, which they categorize into the I and VII groups simultaneously, the atomic number is 1, the atomic mass is 1.0079. The natural hydrogen has the stable isotopes, namely, protium (1H), deuterium (2H, or D), and the radioactive isotope, namely, tritium (3H, or T). For the natural compounds of the Earth, the average ratio D/H = (158±2)•10^-6. The equilibrium content of the 3H on the Earth is ~5•10^27 atoms.

The English scientist Henry Cavendish has described hydrogen for the first time during the 1766. Under the usual conditions, hydrogen is the gas without the colour, smell, and taste. Within the nature, hydrogen exists in the free state in the form of the H2 molecules. The energy of dissociation of the H2 molecule is 4.776 electron-Volts; the potential of ionization of the atom of hydrogen is 13.595 electron-Volts. Hydrogen is the most lightweight substance among all the known substances, the density at 0 degrees Celsius and 0.1 megapascals is 0.0899 kilograms per cubic metre; the temperature of the boiling point is minus 252.6 degrees Celsius, the temperature of the melting point is minus 259.1 degrees Celsius; the critical parameters are: the critical temperature is minus 240 degrees Celsius, the critical pressure is 1.28 megapascals, the critical density is 31.2 kilograms per cubic metre. Hydrogen is the most heat-conducting gas among all the gases, namely, 0.174 Watts per (metre•Kelvin) at 0 degrees Celsius and 1 megapascals, the specific heat capacity is 14.208•10^3 Joules per (kilogram•Kelvin).

The liquid hydrogen is very lightweight (the density at the temperature of minus 253 degrees Celsius is 70.8 kilograms per cubic metre) and fluid (the viscosity at the temperature of minus 253 degrees Celsius is 13.8 centipoises). Within the majority of the compounds, hydrogen manifests the +1 state of oxidation (similar to the alkali metals), and less often the -1 state of oxidation (similar to the hydrides of the metals). Under the usual conditions, the molecular hydrogen is not very active; the solubility within the water at 20 degrees Celsius and 1 megapascals is 0.0182 millilitres per gram; hydrogen is well soluble within the metals, namely, Ni, Pt, Pd, and others. Hydrogen, together with oxygen, forms the water, with the yield of the heat of 143.3 megajoules per kilogram (at 25 degrees Celsius and 0.1 megapascals); at the temperature of 550 degrees Celsius and above, this reaction is accompanied by the explosion. In case of the interaction with fluorine and chlorine, the reactions also proceed with the explosion. The major compounds of hydrogen are: water H2O, ammonia NH3, hydrogen sulphide H2S, methane CH4, hydrides of metals and halogens CaH2, HBr, HI, and also the organic compounds C2H4, HCHO, CH3OH, and others.

Hydrogen is the element, which is widespread within the nature, the content of hydrogen within the Earth's crust is 1% (in terms of the mass). The main reservoir of hydrogen on the Earth is the water within the hydrosphere (11.19%, in terms of the mass). Hydrogen is one of the major components of all the natural organic compounds. In the free state, hydrogen exists within the volcanic and other natural gases, and within the atmosphere (0.0001%, in terms of the quantity of the atoms). Hydrogen constitutes the major part of the mass of the sun, stars, interstellar gas, and gaseous nebulae. Within the atmospheres of the planets, hydrogen exists in the form of the H2, CH4, NH3, H2O, CH, NHOH, and other molecules. Hydrogen belongs to the composition of the corpuscular radiation of the sun (the streams of protons), and of the cosmic rays (the streams of electrons).

The raw materials for the industrial obtainment of hydrogen are the gases from the processing of the petroleum, the natural gases, the products from the gasification of the coal, and others. The major methods for the obtainment of hydrogen are: the reaction of the hydrocarbons with the water vapour, the partial oxidation of the hydrocarbons by oxygen, the conversion of the carbon monoxide, the electrolysis of the water. They use hydrogen for the production of ammonia, alcohols, synthetic gasoline, hydrochloric acid, for the hydraulic purification of the petroleum products, for the cutting of the metals using the hydrogen-oxygen flame.

Hydrogen is the prospective gaseous fuel. Deuterium and tritium have found the usage within the nuclear energetic industry.