nuclear-physics
according to this image of nuclear fission, before fisson reaction, the total mass of uranium and the slow neutron is 236 amu. and after adding the masses of Barium, krypton and the 3 neutrons after the fission reaction, you will again get 236 amu.
Question: if we get back all the mass, where did energy come from? is it the binding energy that is released?
They don't. Here's a figure from Wikipedia:
Typically there's daughter with mass around 95, a daughter with mass around 140, and two or three extra free neutrons. In discussion of environmental contamination after nuclear accidents, you hear a lot about iodine-133 and strontium-90, because they are relatively long-lived and biologically active. Iodine-133 lives for about a week and accumulates in the thyroid; strontium-90 lives for about 30 years and can replace calcium in bones.
There are several heavy isotopes which can spontaneously fission; the big ones are uranium, plutonium, and californium.
In principle the energy released is the total energy in (mass of $^{235}U$ + energy of thermal neutron) minus the total rest mass of the fission products. In practice, the neutron coming in is thermal (moderated), and has an energy of a few eV, which we can neglect. This means we can compute the energy released from the difference in the rest masses of the "in" and "out" particles. The rest mass of $^{236}U$ is not relevant, because what is formed is an excited state.
Best Answer
Look up the masses to high precision (i.e. don't just count nucleons). You don't get back all the mass.
The mass of $ ^{235}\mathrm{U}$ is 235.0439299 u not 235 u and so on.