Lasers can heat a tiny sample of a hydrogen isotope enough to begin nuclear fusion.
Explain what makes two atoms isotopes of one element.
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Isotopes are atoms of one element whose nuclei hold equal numbers of protons yet unequal numbers of neutrons, so their mass numbers differ.
How the marks are earned
- One mark: isotopes share a proton count, which is their atomic number, so they are the same element.
- One mark: their neutron counts differ, which changes the mass number. Saying a different mass number also earns this.
- Electron numbers earn nothing.
Why it scores
Both halves are needed. Proton number fixes the element, and the neutron difference is what makes them isotopes.
Revise Atomic StructureDescribe the process of fusion and say where the energy it releases comes from.
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Two light nuclei, such as hydrogen nuclei, are forced together at very high temperature and join to make a heavier nucleus, such as helium.
The new nucleus has slightly less mass than the nuclei that joined. The missing mass is converted into energy, which is released as radiation.
How the marks are earned
- One mark: light nuclei, such as hydrogen nuclei, fuse, which means they join into one nucleus. Saying only that they collide or bond is not enough. A very high temperature is extra detail and is not required.
- One mark: the nucleus produced is heavier than either starting nucleus, for example helium.
- One mark: the product has slightly less mass, and that mass has become energy. Saying the energy is radiation is extra detail, not required.
Why it scores
This covers the first two points: what joins and what forms.
The mass to energy conversion is what actually releases the energy, so it is the final mark.
Revise Nuclear Fission & FusionFusion in a reactor releases huge amounts of energy. Explain why the reactor's walls need an extremely high specific heat capacity.
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Specific heat capacity is the energy needed to raise the temperature of 1 kg of a material by 1 °C. When it is very high, the walls can take in huge amounts of energy yet warm only slightly. Their temperature therefore stays below their melting point.
How the marks are earned
- One mark: a huge energy input produces only a small temperature rise in the walls.
- One mark: the walls therefore stay below their melting point. Saying they should not melt also earns this.
Why it scores
The link is energy to small temperature rise in the walls to staying below the melting point. Each step earns a mark, so the chain must be complete.
Revise Specific Heat CapacityFission can run out of control as a chain reaction. State one possible result.
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A meltdown of the reactor.
How the marks are earned
- Accept one serious outcome of a chain reaction running out of control: the reactor core melting down, a nuclear blast or bomb, or the Chernobyl or Fukushima disaster.