What is binding energy and mass defect?
Binding energy and mass defect are the same, they hold atomic nuclei together.
The atomic nucleus for every element heavier than Hydrogen requires that the strong nuclear force is able to overcome the electromagnetic force. Every proton in a nucleus repels every other proton.
The electromagnetic force is long ranged so in a heavy element such as Uranium every one of the 92 protons repels the other 91.
The strong nuclear force binds adjacent nucleons, which are protons and neutrons, together and is very short ranged. So, how does it overcome the electromagnetic repulsion?
The answer is that bound nucleons have less mass than their free counterparts. A Helium-4 nucleus has less mass than the mass of 2 free protons and 2 free neutrons. This is the mass defect. As mass and energy are equivalent
A Helium-4 nucleus has a particularly high binding energy. Even though the two protons repel each other, the nucleus can only break apart if the binding energy is added.
Only certain combinations of protons and neutrons are stable. There is no combination of 5 protons and neutrons which can stay bound. The same is true for elements heavier than lead.