Helium (He) is a colorless, odorless, unreactive gaseous element which is the second most abundant and second lightest element in the universe
(after hydrogen). It has the lowest melting
point and boiling point of any element. Helium is one of the noble
gases (also known as noble gases). An ordinary helium atom consists
of a nucleus of two protons and two neutrons surrounded by two electrons. Seven isotopes of helium are known.
|The helium nucleus consists of two protons (1) and
(most usually) two neutrons (2), about which two electrons (3) revolve.
The helium atom is smaller in size than the lighter hydrogen atom
with its single proton and electron, because the greater charge of
the helium nucleus pulls the electrons closer in. Although it is easier
to conceive of electrons orbiting the nucleus in circular orbits,
the shell should be considered as a "probability cloud"
of locations in which the electrons are likely to be found. Isotopes
of helium, although retaining 2 protons and 2 electrons, may contain
1 to 4 neutrons.
|A prototype helium-filled Venus balloon in a JPL
cleanroom. Image credit: NASA/JPL-Caltech
The main source of helium is natural gas.
It is used to inflate and provide lift for balloons and airships; in breathing mixtures for
deep-sea divers; as a pressurizer for the fuel tanks of liquid-fueled rockets;
in helium-neon lasers; as an inert gas shield
for arc welding; as a protective gas in
growing silicon and germanium crystals and producing titanium and zirconium;
and as a superfluid in the form of helium II.
Liquid helium 4He exists in two forms. Helium I, stable from
2.19K to 4.22K, is a normal liquid, used as refrigerant and in cryogenics.
Below 2.18K, it becomes helium II, which is a superfluid displaying superconductivity,
zero viscosity, the ability to flow over
the side of a vessel in which it is placed, and other strange properties
predicted by quantum mechanics.
3He does not form a superfluid. Solid helium can only be produced
at pressures above 25 atm.
|Liquid helium cooled below its boiling-point behaves strangely. If a tube is dipped into liquid helium [A] at a temperature of 2.18K (-270.97°C) an invisible film of liquid creeps up the outside of the tube and then down the inside. The helium fills the tube until the liquid levels are the same inside and out. If the tube is raised a little [B] the "superfluid" helium flows the other way to equalize the levels. Drops of the liquid drip off the bottom of the tube. The thickness of the liquid film can be measured (by light polarization) as about 3 millionths of a centimeter at a height of 1cm above the liquid.
Discovery of helium
Jules Janssen obtained the first evidence
of helium during the solar eclipse of 1868 when he detected a new line in the solar spectrum. J. Norman Lockyer and Edward Frankland suggested the name
helium (from the Greek helios for Sun) for the new element. In
1895 William Ramsay discovered helium in
the uranium mineral cleveite; it was independently
discovered in cleveite by the Swedish chemists Cleve and Langlet at about
the same time. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei.
Cosmic significance of helium
Most of the helium in the universe was produced immediately after the Big
Bang, although an additional contribution has come from hydrogen
burning inside main sequence stars.
It occurs commonly in stars and in the atmospheres of gas
giants. See also elements, cosmic abundance.
The helium content of Earth's atmosphere is about 1 part in 200,000.
|relative atomic mass
|| 0.176 g/dm3