Question

Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

Hint:

To remember the distinction: "Fission" sounds like "fissure" (a split or crack), meaning breaking apart. "Fusion" means "fusing" things together into a single whole. Both processes release energy because the final products have a higher binding energy per nucleon than the initial reactants.

Answer 1

Step 1: Energy source of nuclear reactions:
The binding energy per nucleon is highest near mass number 56 (iron region).
Very heavy nuclei move toward it by splitting. Very light nuclei move toward it by joining.
This gain in binding energy appears as released energy.

Step 2: Compare the two processes point by point:
(a) Nuclear fission: One heavy nucleus ($A > 200$) breaks into two medium-mass fragments plus a few neutrons.
(b) Nuclear fusion: Two light nuclei ($A < 20$) merge into one heavier nucleus.
(c) Conditions: Fission is started by a slow neutron at ordinary temperature. Fusion needs about $10^{7}$ K and high pressure, as in the Sun.
(d) Control: Fission gives a chain reaction through the neutrons released. Fusion has no chain reaction.
(e) Energy: A fission event gives about 200 MeV per nucleus. Fusion gives less per event but more per unit mass of fuel.
(f) By-products: Fission fragments are radioactive. Fusion products are mostly stable.

Step 3: Fission example with a balance check:
\[ {}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \longrightarrow {}^{140}_{54}\text{Xe} + {}^{94}_{38}\text{Sr} + 2\,{}^{1}_{0}\text{n} + \text{energy} \]
Mass number: $235 + 1 = 236$ and $140 + 94 + 2 = 236$.
Charge number: $92 = 54 + 38$.
Both are balanced, so the equation is correct.

Step 4: Fusion example with a balance check:
\[ {}^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \longrightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n} + 17.6\ \text{MeV} \]
Mass number: $2 + 3 = 5$ and $4 + 1 = 5$.
Charge number: $1 + 1 = 2$ and $2 + 0 = 2$.
Both are balanced, so the equation is correct.

Final Answer:
Fission: a heavy nucleus splits (example: $^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \to {}^{140}_{54}\text{Xe} + {}^{94}_{38}\text{Sr} + 2\,{}^{1}_{0}\text{n}$). Fusion: light nuclei join (example: $^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \to {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n} + 17.6$ MeV).