Historical
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
Like most gaseous elements, hydrogen is diatomic
(its molecules contain two atoms), but it dissociates into free atoms at
high temperatures. The atomic weight of hydrogen is 1.008.
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; the distinguishing characteristic of an acid is its dissociation, upon going into solution, to yield hydrogen ions.
Uses
Hydrogen reacts with many nonmetals. It combines with nitrogen in the
presence of a catalyst to form ammonia (see Nitrogen Fixation), with sulfur
to form hydrogen sulfide, with chlorine to form hydrogen chloride, and
with oxygen to form water. 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, such as copper oxide, removing the oxygen and
leaving the metal in a free state. Hydrogen reacts with unsaturated organic
compounds to form corresponding saturated compounds.
Hydrogen is prepared in the laboratory by the action of dilute acid
on metals, such as zinc, and by the electrolysis of water.
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, including the Hindenburg, 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 also used
in high-temperature torches for cutting, melting, and welding metals.