Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

The energy required to remove an electron completely from the atom is calledits binding energy. By convention, binding energies are negative with increasingmagnitude for electrons in shells closer to the nucleus. For an electron to becomeionized, the energy transferred to it from a photon or particulate form of ionizingradiation must equal or exceed the magnitude of the electron's binding energy. Thebinding energy of electrons in a particular orbit increases with the number of protonsin the nucleus (i.e., atomic number). In Fig. 2-5, binding energies are comparedfor electrons in hydrogen (Z = 1) and tungsten (Z = 74). If a free (unbound)electron is assumed to have a total energy of zero, the total energy of a bound electronis zero minus its binding energy. A K shell electron of tungsten is much moretightly bound (-69,500 eV) than the K shell electron of hydrogen (-13.5 eV). Theenergy required to move an electron from the innermost electron orbit (K shell) tothe next orbit (L shell) is the difference between the binding energies of the twoorbits (i.e., EbK - EbL equals the transition energy).Hydrogen:Advances in atomic physics and quantum mechanics have led to refinements ofthe Bohr model. According to contemporary views on atomic structure, orbitalelectrons are assigned probabilities for occupying any location within the atom.Nevertheless, the greatest probabilities are associated with Bohr's original atomicradii. The outer electron shell of an atom, the valence shell, determines the chemicalproperties of the element.i ./$'~>- ....Ol •(jjc:Wi ./$' -11,000~>- ....Ol •(jjc:WFIGURE 2-5. Energy-level diagrams for hydrogen and tungsten. Energies associated with variouselectron orbits (not drawn to scale) increase with Z and decrease with distance from thenucleus.