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.


Solid, the physical state of matter in which samples maintain their shape and size. Some highly viscous liquids, such as cold molasses, flow so slowly that they seem to retain their size and shape and thus appear to be solids. X-ray examination, however, reveals an important difference in microscopic structure. Solids exhibit a regular arrangement of atomic, ionic, or molecular particles—solid objects have a crystalline structure. In contrast, the molecules of liquids are arranged irregularly—liquids have no crystalline structure. See States of Matter.