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Neon atom. Computer artwork of electron orbitals in a neon atom. The nucleus is represented by a flash of light. The orbitals shown are 1s (small white sphere), 2s (large black sphere) and 2p (blue lobes). According to quantum theory the position of an electron in an atom can never be precisely defined. Instead, an electron orbital is a region around an atomic nucleus in which one, or a pair of electrons, is most likely to exist. (KEYSTONE/SCIENCE PHOTO LIBRARY/MEHAU KULYK)