Thulium
. Thulium, symbol Tm, silver-gray metallic element that is the rarest of the rare earth elements. Thulium is in the lanthanide series of the periodic table ; the atomic number of thulium is 69. Thulim was discovered in 1879 by the Swedish chemist Per Teodor Cleve. Thulium ranks 61st in abundance among the elements in the crust of the earth and is found in small quantities in such rare earth minerals as euxenite, gadolinite, and blomstrandine. The metal can be isolated by reduction of its oxide, and is soft, malleable, and ductile. Thulium had little practical application until the development in the 1950s of a small, portable X-ray machine that utilizes artificially radioactive thulium as its X-ray source, Thulium melts at about 1545° C (about 2813° F), boils at about 1950° C (about 3542° F), and has a specific gravity of 9.34. The atomic weight of thulium is 168.934.