Introductionhelium (hēˈlēəm) [key], gaseous chemical element; symbol He; at. no. 2; at. wt. 4.0026; m.p. below - 272°C at 26 atmospheres pressure; b.p. - 268.934°C 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. 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) from a sample of the uranium mineral cleveite by Sir William Ramsay.
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