Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Promethium, symbol Pm, radioactive metallic element with an atomic number of 61. Promethium is one of the rare earth elements in the lanthanide series of the periodic table.

Promethium was one of the last elements to be identified. In 1926 evidence from spectroscopic analysis indicated the existence of the element in various minerals, and the names illinium and florentium were proposed for the element. The fission of uranium is known to produce several radioactive isotopes with atomic number 61. These isotopes were investigated and isolated in 1945 by scientists at the nuclear-research laboratory at Oak Ridge, Tennessee; among them were the American chemists Charles DuBois Coryell, Jacob A. Marinsky, and Lawrence E. Glendenin, who proposed the name promethium. Isotopes with mass numbers from 134 to 155 have been investigated. The most stable isotope of promethium, which has a mass number of 145, has a half-life of 2.6 years; visible amounts of this isotope have been prepared. The metal has been used in atomic batteries and as a beta-particle source in thickness gauges. Promethium melts at about 1042° C (about 1908° F), boils at about 3000° C (about 5432° F), and has a specific gravity of 7.26.

See also Beta Particle; Radioactivity.