a soft silvery metallic element of the rare earth group having no stable isotope; was discovered in radioactive form as a fission product of uranium
Promethium. A radioactive element of the rare earth family of metals. It has the atomic symbol Pm, atomic number 61, and atomic weight 147. It has been used in the construction of atomic batteries, in the preparation of self-luminous compounds, and as a beta-particle source for thickness gauges.