Properties of Hydrogen :: essays research papers

HYDROGEN Hydrogen is a gaseous element, symbol H, usually classed in group 1 (or Ia) of the periodic table Hydrogen melts at –259.2Â ° C (–434.56Â ° F) and boils at –252.77Â ° C (–422.986Â ° F). Hydrogen was confused with other gases until the British chemist Henry Cavendish 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 Properties and Occurrence At ordinary temperatures hydrogen is a colorless, tasteless, and odorless gas, with a density of 0.089 g/liter at 0Â ° C (32Â ° F). It is highly flammable. Like most gaseous elements it is diatomic (its molecules contain two atoms), but it dissociates into free atoms at high temperatures. Hydrogen has a lower boiling point and melting point than any other substance except helium. Liquid hydrogen, first obtained by the British chemist Sir James Dewar in 1898, is colorless (but light blue in thick layers) with sp.gr. 0.070. when allowed to evaporate rapidly under reduced pressure it freezes into a colorless solid. Hydrogen is a mixture of two allotropic 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. Practically pure parahydrogen is obtained by adsorbing ordinary hydrogen on charcoal at about –225Â ° C (about –373Â ° F). 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. Both deuterium and tritium are essential components of nuclear fusion weapons, or hydrogen bombs. 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.