Mass Defect

In physics and chemistry, the mass defect is a term referred to the difference in the mass, between the sum of masses of neutrons, protons, electrons of the atoms and mass of atom itself. Typically, the mass defect is associated with a binding energy between the nucleons. The missing mass is the energy that is released by the formation of the nucleus of the atom. The formula of Einstein, E= mc2 can be used for the calculation of binding energy of the nucleus. According to this formula, with the increase in energy, the mass and inertia are also increased. Similarly, by the removal of energy, the mass is also reduced.


Just like the chemical reactions, nuclear reactions are also accompanied by changes in energy. However, as compare to the chemical reactions, the energy changes in the nuclear reactions are enormous. In fact, in nuclear reactions, the energy changes are so large that they often cause measurable changes in the mass. The energy of the neutron is slightly higher than the mass of a proton. Often these are given in the terms of atomic mass units and it is defined 1/12th of the mass of 12 carbon atom. For the calculation of the mass defect for the nucleus that is having A number of protons and B number of neutrons.

ΔM = Amp + Bmn – Mn

Here mp is indicating the mass of the proton, and mn is showing the mass of the neutron. The mass of the nucleus is denoted by the Mn.

Mass defect is always associated with the nuclear binding energy that is the energy required for splitting the nucleus of an atom to the components. For the nuclei, the binding energy is always a positive number as the energy is required by all the nuclei for separating them to the individual neutrons and protons. The mass defect is the nuclear binding energy, that is accounting for the noticeable and significant differences, between the real mass of the nucleus of an atom and expected mass based on the sum of non-bound components.

By recalling the equation that is relating the mass and energy


Here c is indicating the speed of light. As compared to the sum of individual masses the actual mass is always less. The energy is also having mass but that is removed by the total mass of the constituent particles. This missing mass is known as mass defect. This mass defect occurs in the resulting nucleus, and it represents the release of energy when the nucleus is formed.


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