When the actual measured mass of an atom is compared to the sum of the component subatomic particles (electrons, protons, and neutrons), the mass of the isotope is always smaller. This mass loss or defect has been converted to energy in the formation of the atom. The energy equivalent of the missing mass is known as the total nuclear binding energy and can be calculated from the mass defect. Binding energies are often expressed in electronvolts (). The mass defect can be related to this energy term as
where a mass change of 1 corresponds to an energy release of . Often in such calculations the total mass of an electron (0.0005485 ) and a proton (1.0072765 ) is expressed in terms of the mass of a atom, or 1.0078250 .
The predominant isotope of gold, , has an experimentally determined exact mass of 196.967 . What is the total nuclear binding energy of gold in electronvolts per atom?