Paragraph: The mass of a nucleus _ Z ^ A X is less than the sum of the masses of (A-Z) number of neutrons and Z number of protons in the nucleus. The energy equivalent to the corresponding mass difference is known as the binding energy of the nucleus. A heavy nucleus of mass M can break into two light nuclei of masses m₁ and m₂ only if (m₁+m₂ ) M . Also two light nuclei of masses m₃ and m₄ can undergo complete fusion