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.


Einsteinium, symbol Es, artificially created radioactive element with an atomic number of 99. Einsteinium is one of the transuranium elements in the actinide series of the Periodic table. Isotopes of einsteinium with mass numbers ranging from 243 to 256 are known. The element, named in honor of German-born American physicist Albert Einstein, was discovered in 1952 in the debris produced by a thermonuclear explosion (see Nuclear Weapons). The isotope first identified had an atomic mass of 253 and a half-life of 20 days. Subsequently, the most long-lived of all the known einsteinium isotopes, einsteinium-252, was prepared by irradiating plutonium in a nuclear reactor; however, only small amounts are now being produced (see Radioactivity).