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.


Mendelevium, symbol Md, artificially created radioactive element with an atomic number of 101. Mendelevium is one of the transuranium elements in the actinide series of the periodic table. Named for the Russian chemist Dmitry Mendeleyev, mendelevium-256 was discovered in 1955 at the University of California, Berkeley; it was produced by bombarding einsteinium-253 with alpha particles accelerated in a cyclotron (see Particle Accelerators). The isotope produced had a half-life of about 1.3 hours. The most stable isotope, mendelevium-258, has a half-life of 54 days. See also Radioactivity.