Mass defect:

 Where does all that energy come from?

Nuclear fission reactions release a lot of energy. Where does the energy come from? Well, if you make very accurate measurements of the masses of all the atoms and subatomic particles you start with and all the atoms and subatomic particles you end up with, you find that some mass is “missing.” Matter disappears during the nuclear reaction. This loss of matter is called the mass defect. The missing matter is converted into energy.

You can actually calculate the amount of energy produced during a nuclear reaction with a fairly simple equation developed by Einstein: E = mc². In this equation, E is the amount of energy produced, m is the “missing” mass, or mass defect, and c is the speed of light in a vacuum, which is a rather large number. The speed of light is squared, making that part of the equation a very large number that, even when multiplied by a small amount of mass, yields a large amount of energy.