Hydrogen, symbol H, is reactive, colorless, odorless, and tasteless gaseous element. The atomic number of hydrogen is 1. The element is usually classed in group 1 of the periodic table.

Hydrogen was confused with other gases until the a British chemist demonstrated in 1766 that it was evolved by the action of sulfuric acid on metals and also showed at a later date that it was an independent substance that combined with oxygen to form water. The British chemist Joseph Priestley named the gas “inflammable air” in 1781, and the French chemist Antoine Laurent Lavoisier renamed it hydrogen.

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Like most gaseous elements, hydrogen is diatomic, but it becomes and turns into free atoms at high temperatures. Hydrogen has a lower boiling point and melting point than any other substance except helium. When allowed to evaporate rapidly under reduced pressure, it freezes into a colorless solid.

Hydrogen is a mixture of two different forms, orthohydrogen and parahydrogen, ordinary hydrogen containing about three-fourths of the ortho form and one-fourth of the para form. The melting point and boiling point of the two forms differ slightly from those of ordinary hydrogen.
Hydrogen is known to exist in three isotopic forms. The nucleus of each atom of ordinary hydrogen is composed of one proton. Deuterium, present in ordinary hydrogen to the extent of 0.02 percent, contains one proton and one neutron in the nucleus of each atom and has an atomic mass of two. Tritium, an unstable, radioactive isotope, contains one proton and two neutrons in the nucleus of each atom, and has an atomic mass of three.
Free hydrogen is found only in very small traces in the atmosphere, but solar and stellar spectra show that it is abundant in the sun and other stars, and is, in fact, the most common element in the universe. In combination with other elements it is widely distributed on the earth, where the most important and abundant compound of hydrogen is water, H2O. It is a component of all the constituents of living matter as well as of many minerals. It forms an essential part of all hydrocarbons and a vast variety of other organic substances. All acids contain hydrogen.

Hydrogen reacts with many nonmetals. The reaction of oxygen and hydrogen takes place at room temperature only in the presence of a catalyst such as finely divided platinum. When hydrogen is mixed with air or oxygen and ignited, the mixture explodes. Hydrogen also combines with some metals, such as sodium and lithium, to form hydrides. It acts as a reducing agent on metallic oxides.

Hydrogen is prepared in the laboratory by the action of dilute acid on metals, such as zinc, and by the electrolysis of water. Large quantities of the gas are produced industrially from various fuel gases. Hydrogen is separated from water gas, natural gas, either by liquefaction of the other components of the gas or by catalytic conversion of the carbon monoxide to carbon dioxide, which is easily removed.

In many electrolysis reactions hydrogen is an important by-product. Enormous quantities of hydrogen are used in the manufacture of ammonia and in the synthesis of methyl alcohol. It is an important combustible constituent of fuel. The hydrogenation of oils to produce edible fats, of coal to form synthetic petroleum, and of petroleum oils to enrich the gasoline fraction requires large amounts of hydrogen.The lightest in weight of all gases, hydrogen has been used for the inflation of balloons and dirigibles. It ignites very easily, however, a small spark causing it to burn, and several dirigibles have been destroyed by hydrogen fires. Helium, which has 92 percent of the lifting power of hydrogen and is not inflammable, is used whenever possible. Hydrogen is usually stored in steel cylinders at pressures of 120 to 150 atmospheres. Hydrogen is also used in high-temperature torches for cutting, melting, and welding metals.


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