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.


Ion (chemistry), particle formed when a neutral atom or group of atoms gains or loses one or more electrons. An atom that loses an electron forms a positively charged ion, called a cation; an atom that gains an electron forms a negatively charged ion, called an anion. Atoms can be converted to ions by radiation such as X rays or light of sufficient energy. This kind of radiation is thus called ionizing radiation. See Ionization.