Atomic and Nuclear Physics DOE-HDBK-1019/1-93 MASS DEFECT AND BINDING ENERGY Since 1 amu is equivalent to 931.5 MeV of energy, the binding energy can be calculated using Equation (1-2). (1-2) B.E.      D m                    931.5  MeV 1  amu Example: Calculate the mass defect and binding energy for uranium-235.   One uranium-235 atom has a mass of 235.043924 amu. Solution: Step 1:   Calculate the mass defect using Equation (1-1). D m   Z m_p       m_e    A      Z   m_n    m_atom D m   92 1.007826  amu    235      92   1.008665  amu       235.043924  amu D m     1.91517  amu Step 2:   Use the mass defect and Equation (1-2) to calculate the binding energy. B.E.      D m                    931.5  MeV 1  amu 1.91517  amu                    931.5  MeV 1  amu 1784  MeV Energy  Levels  of  Atoms The electrons that circle the nucleus move in fairly well-defined orbits.  Some of these electrons are  more  tightly  bound  in  the  atom  than  others. For  example,  only  7.38  eV  is  required  to remove  the  outermost  electron  from  a  lead  atom,  while  88,000  eV  is  required  to  remove  the innermost electron.  The process of removing an electron from an atom is called ionization, and the energy required to remove the electron is called the ionization energy. In a neutral atom (number of electrons = Z) it is possible for the electrons to be in a variety of different orbits, each with a different energy level.  The state of lowest energy is the one in which the atom is normally found and is called the ground state.  When the atom possesses more energy than its ground state energy, it is said to be in an excited state. Rev. 0 Page 19 NP-01