Mass Defect
Mass defect is the difference between the predicted mass and the actual mass of an atom's nucleus.
Mass defect refers to the difference in mass between an atom and the sum of the masses of the protons, neutrons, and electrons of the atom. This mass is typically associated with the binding energy between nucleons [ThoughtCo].
Mass defect shows the mass of energy that binds the nucleus. It is the difference between the original nucleus's total mass and its constituents' mass after the new nucleus is formed. In this case, the law of conservation of mass is not directly applicable.
A mass defect is an error in mathematics, physics, or astronomy that causes the mass of an object to differ from that expected by chance when the object is tested as usual. The mass defect in the standard model of particle physics is a systematic deviation from the theory's predictions for the masses of particles that describe the observed world. The deviation results from how the standard model of particle physics has to be extended to deal with the high-energy behaviors of particles. The Standard Model, as it stands, is not a good model of nature. For example, an error in a formula that calculates the mass of an object causes it to be heavier than it should be, which violates the rule that mass and energy are conserved [Unacademy].
Khan Academy https://www.khanacademy.org/science/physics/
Nuclear-Power https://www.nuclear-power.com/nuclear-power/nuclear-energy/mass-defect/
Mass Defect Definition in Physics and Chemistry. ThoughtCo. Retrieved from: https://www.thoughtco.com/definition-of-mass-defect-605328
All about mass defect and binding energy. Unacademy. Retrieved from: https://unacademy.com/content/neet-ug/study-material/physics/all-about-mass-defect-and-binding-energy/