Nuclear fission and fusion
The power in the nucleus
- The nucleus holds a huge amount of energy.
- We release it in two opposite ways: splitting heavy nuclei, or joining light ones.
- These power nuclear stations on Earth and every star in the sky.
Nuclear fission
- In fission, a heavy nucleus absorbs a neutron and splits into two smaller nuclei, releasing 2 or 3 neutrons and a lot of energy.
- Those neutrons can split more nuclei — a chain reaction — which is what a nuclear power station controls.
Nuclear fission is when:
Fission = splitting a heavy nucleus (after it absorbs a neutron) into two smaller nuclei, plus 2–3 neutrons and energy.
Nuclear fusion
- In fusion, two light nuclei join to make a heavier one, also releasing energy.
- This powers the Sun, joining hydrogen nuclei to make helium:
- Fusion needs enormous temperatures and pressures, which is why it is so hard to do on Earth.
Nuclear fusion is when:
Fusion = joining two light nuclei into a heavier one, releasing energy. It powers the Sun.
Which process powers the Sun?
The Sun fuses hydrogen nuclei into helium, releasing huge amounts of energy.
Mass into energy, and balancing
- In both fission and fusion, a tiny amount of mass is lost and turned into energy.
- In any nuclear equation, the top numbers (nucleon numbers) balance, and so do the bottom numbers (proton numbers).
- That lets you find a missing number — for example, how many neutrons are released.
In $^{235}_{\ 92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{\ 56}\text{Ba} + {}^{92}_{36}\text{Kr} + x\,{}^{1}_{0}\text{n}$, how many neutrons $x$ are released?
Nucleon numbers must balance: $235 + 1 = 141 + 92 + x$, so $236 = 233 + x$ and $x = 3$.
In fission and fusion, a small amount of mass is turned into energy.
Both processes lose a tiny amount of mass, which is released as a large amount of energy.
You've got it
- fission: a heavy nucleus splits → two smaller nuclei + neutrons + energy (chain reaction)
- fusion: two light nuclei join → a heavier nucleus + energy (powers the Sun)
- both convert a little mass into energy
- nuclear equations balance top (nucleon) and bottom (proton) numbers