If you add the exact mass of the protons, neutrons, and electrons in an atom you do not get the exact atomic mass of the isotope. The diference is called the mass defect. The difference between the mass of the atomic nucleus and the sum of the masses of the particles within the nucleus is known as the mass defect.

Nuclear binding energy is the energy required to hold the nucleus together. The mass defect is the difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons. The mass defect is converted into nuclear binding energy according to Einstein's famous equation, E=mc^2, where E is the energy, m is the mass defect, and c is the speed of light.

The Energy required o form a nucleus from its parts

nuclear fusion

E = MC2; energy is equal to a quantity of matter.

When protons (and neutrons) combine in an atomic nucleus, the resultant mass is less than that of the individual particles.

This is the mass defect, and the 'missing' mass is a result of the energy binding the particles together. The larger the mass defect for a particular atom (isotope), the larger the amount of nuclear binding energy.