Discovered in 1843, terbium is a soft, ductile, malleable and silvery element of the lanthanide series. Terbium is a rare-earth metal occurring in low concentration in monazite sand, which is a mixture of phosphate of calcium, thorium, cerium, and most of the other rare-earths. This sand is often 50% rare-earth by mass and about 0.03% terbium. Other sources of terbium are oxide mixtures found in xenotime and euxenite. Such oxide mixtures often contain up to 1% terbium.

Terbium is extremely reactive when exposed to air or oxygen and must be kept in an inert atmosphere. Terbium is used in special lasers and as a dopant in solid-state devices.

BCIT Chemistry Resource Center
http://nobel.scas.bcit.ca/resource/