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.


Curium, symbol Cm, radioactive element with an atomic number of 96. Curium is one of the transuranium elements in the actinide series of the periodic table.

Curium is radioactively unstable and does not exist in nature. Curium was first produced synthetically by the American chemists Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944 and was named for Pierre and Marie Curie, research pioneers in radioactivity. The element is made by bombarding the synthetic element plutonium with accelerated particles. Curium is a heavy metal similar in properties to uranium, plutonium, and americium. Curium melts at 1340° C (2444° F) and has a specific gravity estimated at 13.5.

Thirteen isotopes, ranging in mass number from 238 to 250, have been discovered; the most stable isotope of curium has an atomic weight of 247. Most isotopes of curium decay by emission of alpha particles; because alpha radiation is not highly penetrating, curium isotopes, particularly curium-244, can be used without heavy shielding as sources of thermoelectric power for use in satellites and crewless space probes. In another application, curium-242 carried to the moon by the Surveyor 5, 6, and 7 spacecraft was used to bombard the soil of the moon with alpha particles. Measurements of the energy of alpha radiation backscattered from the soil revealed the kind and quantity of many chemical elements in the soil.