Mass Defect

Mass Defect is the release of energy as some mass is converted to energy through the fusion or fission of elements.

It is best described as the release or absorption of energy binding atoms together.
Einstein proposed that energy and mass are equivalent and that the energy contained within matter is equivalent to its mass times the speed of light squared. This startling realisation lead to an understanding of the effects mass defect has on the release of energy of atoms undergoing fusion or fission reactions.

The effect and observations of mass defect:
Mass defect can be clearly seen when measuring the mass of the nucleus vs. the constituent parts.
Let us take for example Deuterium and a proton and neutron.

A proton weighs 1.007276u or 938MeV
A neutron weighs 1.008665u or 940MeV

As a proton and neutron combine a deuterium nucleus and some energy is released.
Deuterium weighs 2.014u or 1876 MeV with the energy being released as 1.8MeV or 0.002u.

This astounding observation shows that the deuterium nucleus now is lighter than the constituent parts. The remaining energy released is equal to that of the energy/mass difference between the constituent parts and the deuterium nucleus.

So why does this happen?

Mass defect is simply the process that undergoes when nuclei are bound together. To be accurate the strong nuclear force is at work with the exchange of pions between the flavour exchanges of quarks of the constituent nucleons. The virtual pi and rho mesons are carries of a residual strong force acting as the mediator for the force for gluons. Mass defect is converted into the binding energy of the nucleons and it requires the exact energy released to break apart the nucleons.

Some students become confused after learning mass defect, especially with the release of energy from fission and fusion.

Questions posed are of the lines: "If mass defect contributes the binding energy of the nucleus, then why does fusion release energy. If the nucleons are bound together, then it would require energy to break the nucleus into its constituent parts."

This line of thinking is true however, it is important to note that when breaking apart the unstable uranium nucleus, the products produced will have a higher binding energy and are vastly more stable than the heavy nucleus. To illustrate this point, the image below shows the process of nuclear fission with regards to mass defect.