It is used to derive the binding energy of the atom because summing the individual particles of an atom leaves a "defect" in another property.
The most precise "direct test" of this concept, by Rainville et al., studied gamma rays emitted by neutron capture in sulfur and silicon. The first experimental verification of this relation occurred in 1932 and was performed using a multistage diode/capacitor voltage multiplier  generally also considered the first real particle accelerator  invented by Cockcroft and Walton. A relativistic variant of this relation is squared and has an additional factor of momentum squared times speed of (*) light squared on its right-hand side. It is used to derive the binding energy of the atom because summing the individual particles of an atom leaves a "defect" in another property. For 10 points, name this concept summarized by the famous equation E equals m c squared.
The most precise "direct test" of this concept, by Rainville et al., studied gamma rays emitted by neutron capture in sulfur and silicon.
mass-energy equivalence [or obvious equivalents, such as E equals m c squared until they are read]