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.


Americium, symbol Am, artificially created, malleable, radioactive metallic element somewhat similar to lead. The atomic number of americium is 95; the element is one of the transuranium elements in the actinide series of the periodic table.

Americium was the fourth transuranic element to be synthesized. It was discovered in 1944 and 1945 by the American physicist Glenn Seaborg and his associates at the University of Chicago. They synthesized the americium isotope of mass number 241 by bombarding plutonium-239 with neutrons. Americium isotopes with mass numbers 237 to 247 have been formed; they are all radioactive, with half-lives of from 0.9 minute (americium-232) to about 7400 years (americium-243) (see Radioactivity). Americium-243 is used as target material in nuclear reactors or particle accelerators for the production of even heavier synthetic elements. Americium melts at about 994° C (about 1821° F) and has a specific gravity of about 14.