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.


Halogens (Greek hals, “salt”; genes, “born”), in chemistry, group of five closely related chemically active elements—fluorine, chlorine, bromine, iodine, and astatine. The name halogen, or salt former, refers to the property of each of the halogens to form with sodium a salt similar to common salt (sodium chloride). Each member of the group has a valence of -1 and combines with metals to form halides, as well as with metals and nonmetals to form complex ions.