The sum of the masses of the particles that make up an atom, such as protons, neutrons, and electrons, is larger than the actual mass of the atom. This phenomenon is known as the "mass defect" or "mass deficiency." The mass defect arises due to the conversion of mass into energy during the formation of atomic nuclei.
According to Albert Einstein's mass-energy equivalence principle (E=mc²), mass and energy are interchangeable. During the process of nuclear formation, a small amount of mass is converted into energy, which is released in the form of binding energy that holds the nucleus together. This conversion of mass into energy follows the famous equation E=mc², where E represents energy, m represents mass, and c represents the speed of light.
The binding energy is a result of the strong nuclear force, which overcomes the electrostatic repulsion between protons in the nucleus. As nucleons (protons and neutrons) come together to form a nucleus, a portion of their mass is converted into binding energy. This conversion reduces the total mass of the nucleus compared to the combined masses of its individual nucleons.
Since electrons are relatively lightweight compared to protons and neutrons, their contribution to the total mass of an atom is negligible. Therefore, the mass defect is primarily attributed to the conversion of mass into binding energy in the atomic nucleus.
It is worth noting that the mass defect is typically a very small fraction of the total mass of an atom. However, on a larger scale, this mass defect can be significant when considering large numbers of atoms, as in the case of nuclear reactions or nuclear power generation.