Dictionary
he•li•um
Pronunciation: (hē'lē-um), [key]—
n.
an inert, gaseous element present in the sun's atmosphere and in natural gas, and also occurring as a radioactive decomposition product, used as a substitute for flammable gases in dirigible balloons.
Symbol: He;
at. wt.: 4.0026;
at. no.: 2;
density: 0.1785 g/l at 0°C and 760 mm pressure.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.