Question

Explain nuclear fission and nuclear fusion with examples.

Hint:

Fission splits a heavy nucleus into two lighter ones; fusion joins two light nuclei into a heavier one. In both the mass defect is released as energy through \(E=\Delta m c^{2}\).

Answer 1

Step 1: What is nuclear fission.
Nuclear fission is the process in which a heavy nucleus (like uranium) splits into two lighter nuclei of nearly comparable masses, when it is struck by a slow (thermal) neutron. In the process a large amount of energy is released along with two or three fresh neutrons.

Step 2: Example of fission.
When a slow neutron hits a \(^{235}_{92}U\) nucleus:
\[ ^{235}_{92}U + ^{1}_{0}n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^{1}_{0}n + Q \]
Here about \(Q \approx 200\) MeV of energy is released per fission. The fresh neutrons can split more uranium nuclei, giving a chain reaction (used in nuclear reactors and atom bombs).

Step 3: What is nuclear fusion.
Nuclear fusion is the process in which two very light nuclei combine (fuse) together to form a single heavier nucleus, with the release of a large amount of energy. It needs extremely high temperature and pressure so that the nuclei can overcome their electrostatic repulsion.

Step 4: Example of fusion.
Fusion of deuterium and tritium:
\[ ^{2}_{1}H + ^{3}_{1}H \rightarrow ^{4}_{2}He + ^{1}_{0}n + 17.6\ \text{MeV} \]
The energy of the Sun and stars comes from such fusion reactions (the proton-proton cycle converting hydrogen into helium).

Step 5: Source of the released energy.
In both processes the total mass of the products is slightly less than the total mass of the reactants. This lost mass \(\Delta m\) (mass defect) is converted into energy according to Einstein relation \[ E = \Delta m\,c^{2} \] which appears as the released nuclear energy.

\[\boxed{\text{Fission: heavy nucleus splits; Fusion: light nuclei combine; both release energy via } E=\Delta m c^{2}}\]