Helium has the distinction of being the only element discovered outside
of Earth prior to finding it within our planet.
The gas was first isolated from terrestrial sources in 1895 by the
British chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearing
mineral.
In 1907 the British physicist Sir Ernest Rutherford showed that alpha
particles are the nuclei of helium atoms, which later investigation confirmed.
Properties and Occurrence
Helium has monatomic molecules, and is the lightest of all gases except
hydrogen. The atomic weight of helium is 4.003.Helium, like the
other noble gases, is chemically inert. Its single electron shell is
filled, making possible reactions with other elements extremely difficult
and the resulting compounds quite unstable.
Helium is the most difficult of all gases to
liquefy and is impossible to solidify at normal atmospheric pressures.
These properties make liquid helium extremely useful as a refrigerant and
for experimental work in producing and measuring temperatures close to
absolute zero. Liquid helium can be cooled almost to absolute zero at
normal pressure by rapid removal of the vapor above the liquid. It
has no freezing point, and its viscosity is apparently zero; it passes
readily through minute cracks and pores and will even creep up the sides
and over the lip of a container.
Helium is the second most abundant
element in the universe, after hydrogen. About 1 part per million
of atmospheric helium consists of helium-3, now thought to be a product
of the decay of tritium, a radioactive hydrogen isotope of mass 3. The
common helium isotope, helium-4, probably comes from radioactive alpha
emitters in rocks. Natural gas, which contains an average of 0.4 percent
helium, is the major commercial source of helium.
Uses
Because it is noncombustible, helium is preferred to hydrogen as the
lifting gas in lighter-than-air balloons; it has 92 percent of the lifting
power of hydrogen, although it weighs twice as much. Helium is used to
pressurize and stiffen the structure of rockets before takeoff and to pressurize
the tanks of liquid hydrogen or other fuel in order to force fuel into
the rocket engines. It is useful for this application because it remains
a gas even at the low temperature of liquid hydrogen. A potential use of
helium is as a heat-transfer medium in nuclear reactors because it remains
chemically inert and nonradioactive under the conditions that exist within
the reactors.
Helium is used in inert-gas arc welding for light
metals such as aluminum and magnesium alloys that might otherwise oxidize;
the helium protects heated parts from attack by air. Helium is used in
place of nitrogen as part of the synthetic atmosphere breathed by deep-sea
divers, caisson workers, and others, because it reduces susceptibility
to the bends. This synthetic atmosphere is also used in medicine to relieve
sufferers of respiratory difficulties because helium moves more easily
than nitrogen through constricted respiratory passages. Helium is transported
as a gas in small quantities, compressed in heavy steel cylinders. Larger
amounts of helium can be shipped as a liquid in insulated containers, thus
saving shipping costs.