The total mass of a nucleus' components – neutrons and protons – is always smaller than the mass of the nucleus in its ground state. Explain.
When nucleons come together to form a nucleus, they are strongly attracted to one another. Their potential energy is dwindling and turning negative. The nucleons in the nucleus are held together by this potential energy. As the potential energy of the nucleon decreases, the mass of the nucleon inside the nucleus decreases as well.