What is a mass defect and why is it important?

Mass defect is the difference in the mass of a nucleus and the sum of the mass of the particles that make up the nucleus.

A nucleus of any atom contains neutrons and protons (only one proton in case of Hydrogen). If the mass of the nucleus of an element is measured and the sum of the mass of the protons and neutrons that make up the nucleus is measured then the mass of the nucleus is found to be slightly lighter. This difference in the mass is known as the mass defect .

Now let us find out where has this mass gone? This has been explained by Albert Einstein who has given the formula as below:

##E=mc^2##

Thus according to this formula the mass defect becomes energy. So where is the energy?

This is the energy required to separate the nucleons (i.e. protons and neutrons) from the nucleus. So when the nucleons are separated the the energy that was required to separated then is converted into mass. The energy which is required to separate the nucleons is called as the Nuclear Binding Energy.