Question

What is nuclear fission? Explain with an example. Write the equation of the nuclear reaction involved.

Answer 1

Hint: We know that nuclear fission is a type of a nuclear reaction that involves splitting of atoms. In such a case, recall how this would be carried out in case of a uranium atom that is bombarded by a neutron, and the subsequent processes that follow through. Remember that fission processes usually involve an exudence of a large amount of energy in various forms. As for the nuclear reaction, ensure that it is balanced at all stages.

Complete answer:
The phenomenon of splitting up of a heavy atomic nucleus into lighter fragments and nuclei along with the emission of gamma rays or photons releasing large amounts of energy is termed as nuclear fission. This process can occur naturally by spontaneous splitting of an atom via radiative decay or can be simulated in a lab by achieving appropriate conditions such as bombarding the heavy atom with subatomic particles like neutrons. In any case, fission processes are exothermic reactions releasing energy in the form of electromagnetic radiation as well as kinetic energy imparted to the produced fragments.
In controlled environments such as reactors, all nuclear fission occurs as a nuclear reaction that is bombardment-driven and involves collision of two subatomic particles. Most reactions involve collision of a subatomic particle with an atomic nucleus that results in the formation of resultant fragments and particles.
Let us look at how this would work with an example of a Uranium atom bombarded by a neutron. This was essentially how nuclear fission was first discovered by Hahn and Strassman in 1939.
They bombarded a uranium nucleus $^{235}U$ with slow neutrons and found that the nucleus split into two medium weight parts with a release of enormous amounts of energy.

When a neutron strikes a $^{235}U$ nucleus, the neutron gets absorbed by it, producing an unstable uranium $^{236}U$ nucleus. This intermediate nucleus gets split into intermediate mass nuclei barium $^{141}Ba$ and krypton $^{92}Kr$. Additionally, three neutrons are ejected out and a small mass defect occurs, which is converted into enormous amounts of energy.
In this process, $0.1\%$ of the mass of the reactants gets converted into energy. This is equivalent to a mass defect of $0.214\;amu$. This mass gets converted to an energy of:
$E = mc^2 = 0.214 \times 931 \approx 200\;MeV$ released per fission of the uranium $^{235}U$ nucleus.
The above reaction can be summarized by a nuclear reaction equation as follows:
${}^{235}U_{92} + {}^{1}n_{0} \rightarrow \left[{}^{236}U_{92}\right] \rightarrow {}^{141}Ba_{56}+{}^{92}Kr_{36}+3{}^{1}n_{0}+Q$
During the fission of uranium two to three neutrons are also released, usually within $10^{-18}\;s$ and $10^{-15}\;s$ and are hence called prompt neutrons. Some neutrons are also released after a significant amount of time and are called delayed neutrons.
The delayed neutrons are further bombarded with more $^{235}U$ following which the whole process repeats as described above, producing neutrons that are again bombarded with uranium nuclei, producing a perpetual chain reaction. The number of neutrons produced increases in geometric progression, which in turn split a larger number of uranium atoms, releasing even more enormous amounts of energy in a short time. If these reactions are uncontrolled, they result in explosive fission processes, like in atom bombs.

Note:
Note that the products of uranium fission are not necessarily always barium and krypton. Sometimes, they are strontium and xenon. In any case, the uranium fission products fall into two groups:
one of atomic number in range 35-43 and atomic mass in range 80-110, and
the other of atomic numbers in range 51-57 and atomic mass in range 125-60.
Also, fission of uranium is also possible with high speed protons of energy $6.9\;MeV$ or by deuterons of energy greater than $8\;MeV$ or by $\alpha$-particles of energy $32\;MeV$