What Is An Atom?
Atoms are the extremely small particles of which we, and everything around us, are made. A single element, such as oxygen, is made up of similar atoms. Different elements, such as oxygen, carbon, and uranium contain different kinds of atoms. There are 92 naturally occurring elements and scientists have made another 17, bringing the total to 109. Atoms are the smallest unit of an element that chemically behaves the same way the element does. When two chemicals react with each other, the reaction takes place between individual atoms--at the atomic level. The processes that cause materials be radioactive--to emit particles and energy--also occur at the atomic level.
In the early 20th century, an English scientist, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom that described an atom as looking very much like our solar system. At the center of every atom was a nucleus, which is comparable to the sun in our solar system. Electrons moved around the nucleus in 'orbits' similar to the way planets move around the sun. (While scientists now know that atomic structure is more complex, the Rutherford-Bohr model is still a useful approximation to begin understanding about atomic structure.)
Opposite electrical charges of the protons and electrons do the work of holding the nucleus and its electrons together. Electrons closer to the nucleus are bound more tightly than the outer electrons because of their distance from the protons in the nucleus. The electrons in the outer orbits, or shells, are more loosely bound and affect an atom's chemical properties. A delicate balance of forces among nuclear particles keeps the nucleus stable. Any change in the number, the arrangement, or energy of the nucleons can upset this balance and cause the nucleus to become unstable or radioactive. (Disruption of electrons in the inner orbits can also cause an atom to emit radiation.) The amount of energy required to break up the nucleus into its parts is called the binding energy; it is often referred to as 'cosmic glue'. This is the same amount of energy given off when the nucleus formed.
EPA US Environmental Protection Agency