(hē´•lē•ėm-3) A stable isotope of the gaseous chemical element; symbol He3; atomic number 2; atomic weight 3.0026; melting point below 272°A at 26 atmospheres pressure; boiling point 268.934°A at 1 atmosphere pressure; density 0.1785 grams per liter at STP ; valence usually 0. Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868 CE. A bright yellow emission line was observed and was later shown to correspond to no known element; the new element was named by J. N. Lockyer and E. Frankland from Helios [Gr., sun]. Helium was isolated (1895 CE) from a sample of the uranium mineral cleveite by Sir William Ramsay.

Helium Isotopes

Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Extremely unreactive, it is an inert gas in group 0 of the periodic table. Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. In helium obtained from natural gas about one atom in 10 million is helium-3. The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons).

Applications & Uses

Helium-3 replaced deuterium (heavy hydrogen) in early fusion reactors in the early Twenty-second Century when the aerostat factories in Jupiter’s upper atmosphere made the isotope economically practical. The switch to helium-3 made fusion power plants reliable. Within the first three decades of the Twenty-second Century fusion power had replaced fossil fuel and hydroelectric plants to meet the growing needs for Earth’s electric power.

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