Pronunciation: (sē'zē-um), [key]
a rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Symbol: Cs; at. wt.: 132.905; at. no.: 55; sp. gr.: 1.9 at 20°C; melts at 28.5°C. Also,caesium.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.