|| A nucleus is specified by its number of protons Z, number of neutrons N, and the mass number A = Z+N.
The nucleons (protons and neutrons) in a nucleus are bound together -- their total energy is less than the total energy of the separated particles. The binding energy is the amount of energy given up when the nucleus is formed.
Plotting the binding energy per nucleon versus the mass number A (Figure 14-01) shows that starting from Hydrogen, nuclei become more stable as there are more protons and neutrons, until Iron. After that, the trend reverses.