When an nucleus splits (scission), it is not simply a matter of that nucleus dividing in half to form two equal daughter nuclei. Instead, two (or more) daughter nuclei are formed with uneven masses, which do not add together to form the mass of the original nucleus. That is, along the way, some mass is lost.

The process of fission is not ordered or regular, and it is uncertain, just like any process. The two daughter nuclei are very rarely of even weight, and it is also rare that more than two are formed, although it is possible. The difference in mass of the two particles is proportional to the energy applied to the original nucleus during fission. Low energy fission creates particles of very different masses, high energies create particles of slightly different masses, and medium level fissions create the most even particles. When graphed, this pattern produces a 'double-humped' figure.

However, daughter nuclei are not the only products of fission. Two or three neutrons are emitted at each scission, and these can bombard more original nuclei, thus creating a chain reaction of constant fission until there are no original-type nuclei left.

Einstein's mass-energy equivalence can be used to explain the disparity between the total mass of the daughter nuclei and that of the original: the missing mass has been turned into a vast amount of energy. For each scission, about 0.09% of the original mass is turned into energy, which is about 200 MeV (mega electron volts) per fission. This compares to 10 eV for each atom of carbon burned as coal!

One can well imagine the massive force this would in an uncontrolled chain reaction. Keep in mind that 200 MeV is for one fission only, and this would be repeated exponentially for each new generation in the chain. It is now obvious how destructive fission weapons can be.