Atoms are the building blocks of the universe. Although there are only about 100 different kinds, their combinations are unlimited. How an atom holds together and is organized determines what it will be -- a gas, solid, diamond or apple, perhaps. Atoms have a nucleus, protons and neutrons, and a cloud comprised of electrons encircling the nucleus in an orbit. Atoms always are in motion, even when they make up an object that looks solid to us, such as a chair.
As energy, such as light, heat or electricity, is applied to atoms as they pass through different states of excitation, and move to an excited level. To put it simply, if an atom is heated, it will go from a low-energy orbit to a higher-energy orbit further away from the nucleus. When an electron moves back from a higher-energy orbit to a low-energy orbit it releases a particle of light, a "photon." The instrument used to control the way light is released by an excited atom is called a laser.
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