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.

Alpha Particle

Alpha Particle, positively charged nuclear particle, symbol a, consisting of two protons bound to two neutrons. Alpha particles are emitted spontaneously in some types of radioactive decay (see Radioactivity). They are also produced when helium-4 atoms are completely ionized (see Ion; see Ionization).