Cesium?
Mirriam Webster
Main Entry: ce·si·um
Pronunciation: 'sE-zE-&m also -zhE-
Function: noun
Etymology: New Latin, from Latin caesius bluish gray
Date: 1861
: a silver-white soft ductile element of the alkali metal group that is the most electropositive element known and that is used especially in photoelectric cells
Most have been one of the radioisotopes - the OP decayed.
Perhaps you meant “cesium”, or maybe “caesium” for our UK friends?
What about the stuff?
Oxford English
Main Entry: caesium (US cesium)
Pronunciation: si-ziam
Function: noun
: a rare silver-white soft element of the alkali metal group used in photoelectric cells and atomic clocks
The server ate UWmite’s OP, so I’ll close this thread and encourage him to try again.