Mass-Energy Relation and Mass Defect

Einstein's special theory of relativity established that mass and energy are not two separate, independently conserved quantities as classical physics had assumed, but are, in fact, two different forms of one and the same physical quantity, related by his famous MASS-ENERGY EQUIVALENCE relation

where $c = 3\times10^8\ \text{m/s}$ is the speed of light in vacuum. This means that a change in the mass of a system corresponds directly to a change in its total energy, and vice versa -- and because $c^2$ is such an enormous number, even a very small change in mass corresponds to a very large amount of energy. This relation is barely noticeable in ordinary chemical reactions, where the mass change involved is far too tiny to detect, but it becomes centrally important inside the nucleus, where the energies involved are millions of times larger.

Careful measurement shows a remarkable fact: the mass of any stable nucleus is always somewhat LESS than the total mass of its individual, separated protons and neutrons added up on their own. This 'missing' mass is called the MASS DEFECT of the nucleus, $\Delta m$, and is defined as

$\Delta m = \left[Z\,m_H + (A-Z)\,m_n\right] - M$ …