According to the law of conservation of energy : energy can neither be created nor destroyed. Thus the energy being released during nuclear decays or transmutation, must either originate within the atom or supplied.
In nuclear decays or transmutation which release energy, the mass of the product(s) is lesser than the sum of the mass of the original constituents. This difference in mass is called as mass defect or .
This mass defect must contribute to something else and simply cannot vanish. This mass actually gets released as energy.
Since the mass of the product(s) is lesser than the sum of the mass of the original constituents, to reverse the process, the difference in mass has to be supplied as energy. Thus it is called the binding energy.
The mass defect gets released as energy. The quantity of energy that gets released from the mass defect is governed by Einstein’s mass-energy equivalence relationship E = mc2 , where mass is actually mass defect = .
Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)