He + 2 positrons + energy
Helium is a nonmetal and the lightest element in Group VIIIA, the rare gas elements. Other members of the group are neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).
The element was discovered by Ramsay in 1896.
In 1888-90, W.F. Hillebrand treated uranium ore with acid, separating a gas but not recognizing it as helium. Sir William Ramsay repeated the experiment using the uranium mineral cleveite, and, in 1896, after identifying the gas evolved, he announced the isolation of helium. It was independently discovered in cleveite by the Swedish chemists Cleve and Langlet at about the same time.
The name is from the Greek word, helios, for the sun since the evidence of the existence of helium was first obtained by Janssen during the solar eclipse of 1868. He detected a new yellow line in the solar spectrum. Lockyer and Frankland suggested the name helium for the new element after determining that the spectral line was due to a previously unknown element, and the symbol He is an abbreviation of the name.
Helium is the second most abundant element in the universe, but its abundance on earth is very low.
Fusion of hydrogen nuclei to give helium is the source of the Sun's energy.
4H He + 2 positrons + energy
The rare gas elements are characterized generally by their chemical inertness. Several do form compounds (Kr, Xe, and perhaps Rn), but He, Ne, and Ar are inert.
Alpha particles that arise in radioactive decay are helium nuclei.
Except for hydrogen, helium is the most abundant element found throughout the universe. It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is an important component in both the proton-proton reaction and the carbon cycle, which accounts for the energy of the sun and stars. The fusion of hydrogen into helium also provides the energy of the hydrogen bomb.
The helium content of the atmosphere is about 1 part in 200,000. However, the element is chemically inert, forming no true compounds.
The alpha rays emanating from a radioactive substance are nuclei of helium atoms. While it is present in various radioactive minerals as a decay product, helium is produced chiefly in Texas and Kansas, where it occurs as a minor constituent of the natural gas produced with oil there. The cost of helium fell from 2500 dollars/cu. ft. in 1915 to 1.5 cents/cu. ft. in 1940. The current price of 99.99% pure helium is about 13 cents/cu. ft. in small quantities. Helium has the lowest melting point of any element and has found wide use in cryogenic research as its boiling point is close to absolute zero. Its use in the study of superconductivity is vital.
A mixture of 80% helium and 20% oxygen is used as an artificial atmosphere for divers and others working under pressure. Since helium is less soluble in the blood than nitrogen and does not produce the "bends," a painful condition caused by the formation of bubbles of gas in the bloodstream when a diver comes to the surface.
In 1961 information obtained through a man-made satellite confirmed the existence of a shell of helium around the earth; starting about 600 miles above the earth's surface, this layer is roughly 900 miles thick.
Four isotopes of helium are known. Liquid helium exists in two forms: He I and He II, with a sharp transition point at 2.186K (at a pressure of 3.83 cm Hg). He I (above this temperature) is a normal liquid, but He II (below it) is unlike any other known substance. It expands on cooling; its conductivity for heat is enormous; and neither its heat conduction nor viscosity obey normal rules. It has other peculiar properties. Helium is the only liquid that cannot be solidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but it can readily be solidified by increasing the pressure.