The smallest possible unit of an element, consisting of a nucleus containing one or more protons and (except hydrogen) one or more neutrons, and one or more electrons which revolve around it. The protons are positively charged, the neutrons have no charge, the electrons are negatively charged. As each atom contains the same number of protons as electrons, the atom is electrically neutral. Atoms in general are characterized by stability. One might wonder why the negatively charged electrons are not attracted into the positively charged nucleus in response to the law of opposite charges, causing the atom to collapse on itself. That this does not occur is due to the nature of the electron, which is not only a particle but also a standing quantum wave. As explained by Dr. W. V. Houston, “The normal state of an atom is balance between the attraction of the nucleus for the electron wave and what might be called the elastic resistance to compression of the wave itself.” Atoms of the various elements differ in mass (weight), that is, in the number of neutrons and protons and also in the number of electrons. Atoms of a given element are identical, except that an element may have atoms of different masses, called isotopes. Individual atoms of uranium and thorium have been resolved at 5 Å in the scanning electron microscope. Motion pictures of uranium atoms at magnification of 7.5 million times have been made at the Enrico Fermi Institute at the University of Chicago. Atoms of the same or different elements combine to form molecules. When the atoms are of two or more different elements, these molecules are called compounds. Atoms remain essentially unchanged in chemical reactions except that some of the outermost electrons may be removed, shared, or transferred as occurs in oxidation, ionization, and chemical bonding. A few atomic species disintegrate as a result of nuclear changes and thus are radioactive. Heavy unstable atoms such as uranium-235 and plutonium can be split by bombardment with high-energy particles yielding tremendous energy.See Electron; Proton; Bond, Chemical; Orbital Theory; Ionization; Radioactivity; Fission.
