Final answer:
The missing mass when uranium-235 fissions is converted into a substantial amount of energy, following the principle of E=mc², where a small amount of lost mass results in a large energy release.
Step-by-step explanation:
When the nucleus of an atom of uranium-235 fissions, some of the mass of the original nucleus is not found in the fission products. This missing mass is converted into energy according to Albert Einstein's famous equation E=mc². In the case of U-235, when it undergoes nuclear fission, the products weigh about 0.2 grams less than the reactants. This lost mass is converted into a very large amount of energy, roughly 1.8 × 10¹⁰ kJ per mole of U-235. It is this mass-to-energy conversion that explains why nuclear fission reactions, such as the fission of U-235, produce such tremendous amounts of energy, far exceeding that of chemical reactions like the burning of coal.