Fission splits a heavy nucleus and fusion joins two light ones. In both, the products have slightly less mass than the reactants, and the missing mass appears as energy.
This lesson is part of SPM Physics nuclear physics. It uses the conservation rules from balancing simple nuclear equations.
How do the two processes compare?
| Feature | Fission | Fusion |
|---|---|---|
| What happens | Heavy nucleus splits | Light nuclei join |
| Trigger | Absorbs a neutron | Very high temperature and pressure |
| Products | Two medium nuclei, neutrons, energy | A heavier nucleus, energy |
| Chain reaction | Yes, from released neutrons | No |
| Found in | Nuclear power stations | Stars |
Worked examples
A fission equation. ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n
Nucleon number: 235 + 1 = 236 and 141 + 92 + 3 = 236. Proton number: 92 + 0 = 92 and 56 + 36 + 0 = 92.
The three neutrons released can each strike another uranium nucleus, which is how a chain reaction continues.
A fusion equation. ²₁H + ³₁H → ⁴₂He + ¹₀n
Nucleon number: 2 + 3 = 5 and 4 + 1 = 5. Proton number: 1 + 1 = 2 and 2 + 0 = 2.
How much energy is involved?
Use E = mc², where m is the mass defect and c = 3.0 × 10⁸ m/s. If 1.0 g (0.0010 kg) of mass were converted to energy, E = 0.0010 × (3.0 × 10⁸)² = 0.0010 × 9.0 × 10¹⁶ = 9.0 × 10¹³ J.
A small mass gives an enormous amount of energy because c² is very large.
The mistake that costs marks
The common slip is to write that mass is conserved, or that fission joins nuclei. The mass of the products is slightly less than the mass of the reactants.
| Statement | Wrong | Right |
|---|---|---|
| Fission | Two nuclei join | A heavy nucleus splits |
| Energy source | Mass is conserved | A small mass defect becomes energy |
A second slip is to say fusion happens in nuclear power stations. Power stations use fission.
Check yourself
Complete ²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + x ¹₀n, find x, and check the proton number.
Answer
Nucleon number: 235 + 1 = 236, so 144 + 89 + x = 236, which gives x = 3.
Proton number: 92 on the left and 56 + 36 = 92 on the right. The equation balances.
What to study next
Continue with discussing applications and risks without handling materials. Test yourself with the nuclear physics practice set.
The units and significant-figure checker helps with answer presentation. If you want a teacher to review your comparison, see online one-to-one Physics tuition.