Nuclear Binding Energy and Mass Defect

Every nucleus (other than ) has a smaller mass than the combined mass of its constituent protons
and neutrons.

BASIC CONCEPT

The mass    of  a   nucleus is  always  less    than    the combined    masses  of  its constituent protons

and neutrons.

The difference is called the mass defect. Scientists had difficulty explaining why this mass defect
occurred until Einstein discovered the equivalence of matter and energy, embodied by the equation
E = mc^2 , where c is the speed of light in vacuum: 3 × 10^8 m/s. Because its value is so big, even a small
amount of mass will, upon conversion, release a large amount of energy. The mass defect is a result
of matter that has been converted to energy. This energy is called the binding energy. This same
amount of energy is needed to break apart the nucleus and separate the protons and neutrons: the
larger the binding energy, the more stable the nucleus. The binding energy per nucleon peaks at
iron, which implies that iron is the most stable atomic nucleus. In general, intermediate-sized nuclei
are more stable than large and small nuclei.