question_answer

Why is the mass of a nucleus always less than the sum of the masses of its  constituents, neutrons and protons?                 

Answer:

                When nucleons approach each other to form a nucleus, they strongly attract each other. Their potential energy decreases and becomes negative. It is this potential energy which holds the nucleons together in the nucleus. The decrease in potential energy results in the decrease in the mass of the nucleons inside the nucleus.