JOHN RAFFO / SCIENCE PHOTO LIBRARY JOHN RAFFO / SCIENCE PHOTO LIBRARY
Computer abstract representation of the structure of an atom. It is formed by a dense central region, the nucleus, in which protons and neutrons (blue) are closely packed together. In the model here represented the nucleus is surrounded by electrons which move around the nucleus in elliptical orbits (green). This is the old, classical atomic model which was supplanted by the quantum mechanics theory. According to the quantum model it is not possible to specify a precise trajectory for the electrons moving around the nucleus. The quantum theory gives instead the distribution of the probability of finding an electron in a given element of space.
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