2. Nucleii of atoms can naturally break up (natural radioactivity) or something makes the nucleii break up (induced or artifical radioactivity).
3. The protons (and neutrons) are held closely together in the nucleus by the binding force. Since protons all have positive charges, one would assume that you needed the binding force (which is a lot of energy since the distance between protons are so small) for the nucleus to be held together. This large amount of energy comes from the loss of a small amount of mass or mass defect (via Einstein's equation, E = mc2) when the atoms is created. Therefore, if you added all the masses of the individual subatomic particles of the atoms (protons,electrons and neutrons) and compared it to the actual mass of the isotope, you would see that the actual masss of the istope is slightly less mass. Hence, when this mass is converted directly into energy, you have the binding energy value.