The known isotopes of helium contain from one to six neutrons, so their mass numbers range from three to eight.
Of these six isotopes, only those with mass numbers of three (helium-3, or He) are stable; all the others are radioactive, decaying very rapidly into other substances.
Pierre Janssen, who detected a bright yellow line in the spectrum of the solar chromosphere during an eclipse in 1868; this line was initially assumed to represent the element sodium.
That same year the English astronomer Sir William Ramsay discovered the existence of helium on Earth in 1895.
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Meteorites and rocks have been analyzed for helium content as a means of dating.This liquid spreads in a thin film over the surface of any substance it touches, and this film flows without friction even against the force of gravity. A liquid mixture of the two isotopes helium-3 and helium-4 separates at temperatures below about 0.8 K (−272.4 °C, or −458.2 °F) into two layers.By contrast, the less plentiful helium-3 forms three distinguishable liquid phases of which two are superfluids. One layer is practically pure helium-3; the other is mostly helium-4 but retains about 6 percent helium-3 even at the lowest temperatures achieved.Adsorption of other gases on cooled, activated charcoal yields 99.995 percent pure helium.Some helium is supplied from liquefaction of air on a large scale; the amount of helium obtainable from 1,000 tons (900 metric tons) of air is about 112 cubic feet (3.17 cubic metres), as measured at room temperature and at normal atmospheric pressure.