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Chemistry Review

     The key to the chemical behavior of atoms lies not only in its number of electrons, but in their arrangement. Although scientists cannot precisely locate the position of any individual electron at a particular time, they can predict where an electron is most likely to be. This volume of space around a nucleus where an electron is most likely to be found is called the shell, or energy level of that electron.
      Atoms can have many electron shells. Atoms with more than two electrons have more than one electron shell. In Figure 1a, the nucleus is shown as a small circle surrounded by spheres. These spheres represent electron shells. In Figure 1b, electrons are shown within the shells, which are depicted here two-dimensionally as circles. Notice that the atom of nitrogen pictured in Figure 1b has electrons occupying two shells. The innermost shell contains two electrons. The second (outermost) shell has five electrons.

Figure 1 Two types of shell models of the same atom.

      Because the energy of electrons increases as their distance from the attractive force of the nucleus increases, the various electron shells of atoms are also called energy levels. Electrons occupying increasingly distant shells from the nucleus have a stepwise in-crease in their levels of energy. The number of protons in the nucleus also influences the energy of electrons at the various energy levels, since the attractive force of the nucleus increases as its number of protons increases.