When matter is converted in a nuclear reaction?
It depends what the “conversion” is. If you try to combine an alpha particle (helium nucleus) with a large nucleus (e.g., uranium), then the resulting nucleus (plutonium) would be slightly heavier than the sum of the alpha + Uranium. The extra energy is absorbed as “binding energy”. If you try to combine four hadrons (let’s say two protons and two neutrons, just to keep matters simple), to produce an alpha particle, then the total mass goes down (one alpha weighs a tiny bit less than the four hadrons); that mass is “converted” into energy which is released, usually as gamma photons. This is how the Sun works ( 4.25 million tonnes of mass PER SECOND is converted) E = m c^2 — From the tone of the question “matter is converted”, I would go with the existing mass disappears and turns into something else (energy). Also, when this was first suggested (and tried) people were quite surprised to see how much energy could be released in this fashion, compared to the chemical reactions that we
