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.

Particles of Matter

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Ordinary matter makes up all the objects and materials familiar to life on Earth, including people, cars, buildings, mountains, air, and clouds. Stars, planets, and other celestial bodies also contain ordinary matter. The fundamental fermions that make up matter fall into two categories: leptons and quarks. Each lepton and quark has an antiparticle partner, with the same mass but opposite charge. Leptons and quarks differ from each other in two main ways: (1) the electric charge they carry and (2) the way they interact with each other and with other particles.

Leptons and quarks each come in 6 varieties. Scientists divide these 12 basic types into 3 groups, called generations. Each generation consists of 2 leptons and 2 quarks. All ordinary matter consists of just the first generation of particles. The particles in the second and third generation tend to be heavier than their counterparts in the first generation. These heavier, higher-generation particles decay, or spontaneously change, into their first generation counterparts. Most of these decays occur very quickly, and the particles in the higher generations exist for an extremely short time (a millionth of a second or less). Particle physicists are still trying to understand the role of the second and third generations in nature.