Question

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

Answer 1

The mass of a nucleus is always less than the sum of the masses of its individual constituents, such as free neutrons and protons, due to a phenomenon known as **mass defect**. This mass defect occurs because when protons and neutrons combine to form a nucleus, some of their mass is converted into energy, which binds the nucleus together. This energy is called the **binding energy** of the nucleus. According to Einstein’s equation E = mc2 , this binding energy corresponds to a loss of mass.
Thus, the total mass of a nucleus is always less than the sum of the masses of the individual free nucleons (protons and neutrons). This is why the mass of a nucleus is always less than the sum of the masses of its constituent particles.