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An atomic theory is a model developed to explain the properties and behaviors of atoms. As with any scientific theory, an atomic theory is based on scientific evidence available at any given time and serves to suggest future lines of research about atoms. The concept of an atom can be traced to debates among Greek philosophers that took place around the sixth century B.C.  One of the questions that interested these thinkers was the nature of matter. A proponent of the ultimate particle concept was the philosopher Democritus(c. 470–c. 380 B.C.  ), who named those particles //atomos.// In Greek, //atomos// means "indivisible." Rutherford's experiments provided him with two important pieces of information. First, most of the alpha particles traveled right through the foil without being deflected at all. This result tells us, Rutherford concluded, that atoms consist mostly of empty space. Second, a few of the alpha particles were deflected at very sharp angles. In fact, some reflected completely backwards and were detected next to the gun from which they were first produced. Rutherford was enormously surprised. The result, he said, was something like shooting a cannon ball at a piece of tissue paper and having the ball bounce back at you. According to Rutherford, the conclusion to be drawn from this result was that the positive charge in an atom must all be packed together in one small region of the atom. He called that region the nucleus of the atom. A sketch of Rutherford's nuclear atom is shown in the figure as well.