WordNet (r) 3.0 (2006):

promethium
    n 1: 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 [syn: promethium,
         Pm, atomic number 61]

The Elements (07Nov00):

promethium
Symbol: Pm
Atomic number: 61
Atomic weight: (147)
Soft silvery metallic element, belongs to the lanthanoids. Pm-147, the
only natural isotope, is radioactive and has a half-life of 252 years.
Eighteen radioisotopes have been produced, but all have very short
half-lives. Found only in nuclear decay waste. Pm-147 is of interest as
a
beta-decay source, however Pm-146 and Pm-148 have to be removed from it
first, as they generate gamma radiation. Discovered by J.A. Marinsky,
L.E. Glendenin and C.D. Coryell in 1947.