What is nuclear fusion? Why does it need a very high temperature? Write the overall reaction by which the Sun produces energy, and give two differences between fission and fusion.

1. 1 · Define fusion. Two light nuclei join to form a heavier nucleus of higher binding energy per nucleon, releasing energy.
2. 2 · Explain the temperature. Nuclei repel each other; only at millions of kelvin are they fast enough to come within range of the nuclear force: thermonuclear fusion.
3. 3 · Write a fusion reaction. ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV.
4. 4 · Write the Sun's overall reaction. 4 ¹₁H + 2e⁻ → ⁴₂He + 2ν + 6γ + 26.7 MeV.
5. 5 · Give two differences from fission.
   • Fission splits a heavy nucleus; fusion joins light nuclei
   • Fission needs a slow neutron; fusion needs very high temperature
   • Fusion gives more energy per nucleon
6. 6 · Real life. Sunlight that grows our crops comes from fusion; India is a partner in ITER.
7. 7 · Close in one line. So, fusion is the energy source of the stars, and both fusion and fission move nuclei towards the peak of the binding energy curve.

Scoring tip: Balance every equation you write and write 'thermonuclear' when you explain the temperature.