In the 1930s scientists bombarded uranium with neutrons. There were four steps on the way. Enrico Fermi's 1934 experiment sent neutrons into uranium; its nuclei absorbed them and changed into other elements, which he took to be heavier than uranium. Also in 1934 Ida Noddack proposed that the products could weigh less than uranium, yet her paper was ignored because it offered neither an experiment nor a theory. Otto Hahn redid Fermi's work in 1939 and found barium, a lighter element, among the products. The same year Lise Meitner worked through the energy changes in Hahn's data and found that the totals balance as a uranium nucleus breaks into barium and one other nucleus, so energy is conserved.
Choose the word for each gap: nuclei that split are ___, and the process is called nuclear ___.
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Unstable fills the first gap.
Fission fills the second gap.
How the marks are earned
- One mark: the first gap is unstable.
- One mark: the second gap is fission.
Why it scores
A large nucleus such as uranium is not held together firmly, so it can break into two smaller nuclei. A stable nucleus would not split.
Fission means splitting a large nucleus into smaller ones. Fusion is the opposite process, joining small nuclei into a larger one.
Revise Nuclear Fission & FusionSuggest why most scientists believed Fermi's idea in 1934, using the timeline in the context.
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Fermi had experimental results to back his idea, while Noddack had none. A nucleus that takes in a neutron gains mass, so it was reasonable to think the new elements were heavier than uranium.
How the marks are earned
- Any one reason: absorbing a neutron makes a nucleus heavier; Fermi had experimental results; Fermi was already well known; Noddack had no data; or attitudes to women in science at that time.
- Saying that Fermi had shown nuclei could be split earns nothing.
Use the timeline to explain why more scientists trusted the nuclear splitting idea by 1940.
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By 1939 Hahn had run Fermi's experiment again and got a result that others could check. Fresh experiments gave more evidence than the single 1934 result.
Hahn found barium among the products, a lighter element than uranium, and Meitner's calculation showed that the energy balances when uranium splits into barium and another nucleus.
How the marks are earned
- One mark: there was fresh evidence, since Hahn's repeat of Fermi's experiment showed the result could be reproduced, or further experiments tested the idea.
- One mark: one piece of support from the timeline: barium, a lighter element than uranium, was found; or Meitner's calculation showed that energy is conserved in the split.
- Saying that science keeps developing, or only that energy is conserved, earns nothing.
Why it scores
An idea becomes trusted when other scientists can repeat the experiment and find the same thing. This earns the first mark.
Barium could only appear if uranium nuclei had split, and the energy calculation showed that the splitting obeyed a law that must hold. Either piece of evidence earns the second mark.
Revise Nuclear Fission & FusionPick the account of how uranium splitting sets off a self-sustaining chain of splits.
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The first box. Each splitting nucleus gives out neutrons, and each of those can be absorbed by another uranium nucleus and make it split too, so the reaction keeps itself going. Neutrons do not link together, and spreading harmful material is a separate hazard, not the mechanism of the chain.
How the marks are earned
- Tick the box about further uranium nuclei taking in the neutrons given out, and then splitting in their turn.
- One box only: two ticks earn nothing.
Choose two statements on how a splitting uranium nucleus gives out its energy.
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The new particles fly apart at high speed, so the energy appears as their kinetic energy. The splitting also gives out ionising radiation, such as gamma rays.
How the marks are earned
- Tick exactly two boxes. One mark for the box about the movement energy of the new particles.
- One mark for the box about giving out ionising radiation.
Why it scores
Each correct box earns one mark. The original nuclei no longer exist after the split, so they cannot carry the energy. Stored energy and infrared are not how the energy leaves the split.
Revise Nuclear Fission & Fusion