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.


Californium, symbol Cf, artificially created radioactive element with an atomic number of 98. Californium is one of the transuranium elements in the actinide series of the periodic table. The isotope of californium with a mass number of 245 was first produced in 1950 at the University of California laboratories in Berkeley by the American chemists Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiroso, and Glenn T. Seaborg. The scientists created californium-245 by bombarding curium-242 with alpha particles in a 152-cm (60-in) cyclotron (see Particle Accelerator). Californium-245 rapidly decays, with the emission of alpha particles, having a half-life of 44 min. Isotopes, with mass numbers from 240 to 255, were subsequently prepared. Californium-249 is the result of beta decay of berkelium-249. The heavier californium isotopes are produced by neutron bombardment of berkelium-249, which increases the number of protons in the nucleus. Californium-252, with a half-life of 2.6 years, has an unusually high rate of spontaneous fission, with an abundant emission of neutrons. It has practical application as a high-intensity neutron source in electronic systems and in medical research. The most stable isotope of californium, with a half-life of about 900 years, has a mass number of 251. See also Radioactivity.