Physics · nuclei · NEET
Look at the binding energy per nucleon graph. A heavy nucleus like U-235 (A around 240) has E_bn of about 7.6 MeV. When it splits into two medium nuclei (A around 120), their E_bn rises to about 8.5 MeV. The products are more tightly bound, so they have LESS mass than the parent. That missing mass turns into energy through E = mc^2. So energy is released because the system moves to a more stable, more tightly bound state.
Take a parent with A = 240 splitting into two fragments of A = 120 each. Energy released = (total binding energy of products) minus (binding energy of parent). That equals 240 x (E_bn of product minus E_bn of parent) = 240 x (8.5 minus 7.6) = 240 x 0.9 = 216 MeV, which is of the order of 200 MeV. This whole amount first appears as kinetic energy of the fragments and neutrons, then becomes heat.
For neutron-induced fission (the type in reactors and bombs), yes. A slow neutron is absorbed by U-235 to form U-236, which is unstable and splits. Note the incoming neutron carries almost no energy, yet ~200 MeV comes out. This is because the energy was already stored in the nucleus as low binding energy; the neutron just triggers the release. A few nuclei also fission spontaneously, but the NEET syllabus focuses on neutron-induced fission.
The fragments (like Ba-144, Kr-89) are neutron-rich, meaning they have too many neutrons for stability. To fix this, each fragment undergoes beta-minus decay in succession (a neutron converts to a proton, emitting an electron) until it reaches a stable nucleus. So fission is followed by radioactive decay of the products.
No. Fission is a statistical process, so the split varies. NCERT gives three example paths for the same reaction: Ba-144 + Kr-89 + 3n, Sb-133 + Nb-99 + 4n, and Xe-140 + Sr-94 + 2n. In every case the products are medium-mass nuclei plus 2 or 3 (sometimes more) neutrons, and roughly 200 MeV is released.
It is the splitting of a heavy nucleus (such as U-235) into two lighter nuclei plus a few neutrons, with a large release of energy (~200 MeV per fission).
n(0,1) + U(235,92) goes to U(236,92) which then goes to Ba(144,56) + Kr(89,36) + 3 n(0,1). The intermediate U-236 is unstable and splits.
About 200 MeV per fissioning nucleus. Fission of 1 kg of uranium releases about 10^14 J, roughly a million times more than burning 1 kg of coal (about 10^7 J).
Slow neutrons have a much higher probability of being absorbed by U-235 to cause fission. That is why reactors use a moderator (like heavy water or graphite) to slow neutrons down.
It first appears as kinetic energy of the fragments and emitted neutrons, then is transferred to surrounding matter as heat. In a reactor this heat makes steam to generate electricity; in a bomb it is an uncontrolled release.
