Atom


 * Atom:** an atom is the basic unit of matter, it is also the basic unit of chemistry


 * a.** A part or particle considered to be an irreducible constituent of a specified system.
 * b.** The irreducible, indestructible material unit postulated by ancient atomism.

In a series of experiments from 1909 to 1911, Ernest Rutherford established that atoms have nuclei.The radius of an atom’s nucleus is about 1⁄10,000 the radius of the atom itself.
 * Composition:**In the atom there exists a nucleus at the center made up of positively charged particles called protons and electrically neutral atoms called neutrons. Surrounding or orbiting this nucleus are the electrons. In elements the number of electrons equals the number of protons. The atoms weight is roughly the number of neutrons + the number of protons. Since the number of electrons are always the same for specific types of atoms the number of protons will stay constant. This means that the only thing influencing the weight of different substance's isomers is the varying number of neutrons.
 * History:**The first major discovery that set off modern atomic theory was that atoms aren’t in fact the smallest things that exist. J. J. Thompson discovered the electron in 1897.

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Bellow is the picture of atoms of gold produced by electronic microscope.