Physics • Year 12 • Module 8 • Lesson 7
Nucleosynthesis and the Origin of Elements
Lock in the core vocabulary, the nucleosynthetic sites, and the key features of the binding energy curve before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: Big Bang nucleosynthesis, stellar nucleosynthesis, r-process, s-process, mass defect, binding energy per nucleon, iron-56, neutron capture, kilonova, triple-alpha process. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | Production of hydrogen, helium-4, and trace lithium-7 in the first three minutes after the Big Bang. | |
| 1.2 | Fusion of progressively heavier elements within stellar cores and shells during a star’s lifetime. | |
| 1.3 | Rapid neutron capture in extreme neutron-flux environments such as supernova explosions and neutron star mergers, building nuclei far from stability. | |
| 1.4 | Slow neutron capture in asymptotic giant branch stars, with time for beta decay between successive captures. | |
| 1.5 | The difference between the total mass of free nucleons and the actual mass of the nucleus; related to energy released by $E = \Delta m c^2$. | |
| 1.6 | The energy required to remove one nucleon from the nucleus, which peaks at mass number 56. | |
| 1.7 | The nucleus with the highest binding energy per nucleon (~8.8 MeV), representing the most stable nuclide. | |
| 1.8 | Absorption of a neutron by a nucleus; the mechanism by which elements heavier than iron are produced. | |
| 1.9 | A luminous transient caused by r-process nucleosynthesis following a neutron star merger; confirmed by GW170817 in 2017. | |
| 1.10 | The helium-fusing process in stellar cores: three helium-4 nuclei successively fuse to form carbon-12, then oxygen-16. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Big Bang nucleosynthesis produced approximately equal masses of hydrogen and helium. T / F
2.2 Fusion reactions always release energy regardless of which elements are fusing. T / F
2.3 Gold and uranium are primarily produced by the r-process in supernovae and neutron star mergers. T / F
2.4 The binding energy curve shows that iron-56 is the least stable of all nuclei. T / F
2.5 The s-process occurs in low- to intermediate-mass stars during the asymptotic giant branch phase. T / F
2.6 The gravitational wave event GW170817 confirmed that neutron star mergers are a major site of s-process nucleosynthesis. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 9 marks (1 per blank)
Word bank:
absorbed · beta decay · Big Bang · binding energy · endothermic · iron-56 · mass defect · neutron capture · 25
In the early universe, ___________ nucleosynthesis produced roughly 75% hydrogen and ___________% helium by mass. The stability of a nucleus is quantified by its ___________ per nucleon, which peaks at ___________. Fusion of nuclei lighter than iron releases energy because the products have a higher ___________ per nucleon. Fusing nuclei heavier than iron is ___________ because energy must be ___________. Elements heavier than iron are therefore built by ___________, where a neutron is absorbed and subsequent ___________ converts a neutron to a proton, increasing the atomic number.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 Why did Big Bang nucleosynthesis stop after only about three minutes, producing no elements heavier than lithium?
4.2 Why does fusing nuclei beyond iron require energy input rather than releasing energy?
4.3 What is the key difference between the r-process and the s-process in terms of neutron flux and the stability of intermediate nuclei?
4.4 What does the mass defect of a nucleus represent, and how is it connected to the energy released when the nucleus forms?
5. Build a concept map
Draw labelled arrows between the seven terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “produces”, “is site of”, “limits”). Aim for at least 7 labelled arrows. 7 marks (1 per valid labelled arrow)
Supplied terms: Big Bang · stellar nucleosynthesis · iron-56 · r-process · binding energy curve · neutron star merger · gold.
Q1 — Term–definition match
1.1 Big Bang nucleosynthesis • 1.2 stellar nucleosynthesis • 1.3 r-process • 1.4 s-process • 1.5 mass defect • 1.6 binding energy per nucleon • 1.7 iron-56 • 1.8 neutron capture • 1.9 kilonova • 1.10 triple-alpha process.
Q2 — True / false with correction
2.1 False. BBN produced approximately 75% hydrogen and 25% helium by mass — not equal masses. Hydrogen was roughly three times more abundant than helium.
2.2 False. Fusion only releases energy when the product nucleus has a higher binding energy per nucleon than the reactants. Fusing nuclei heavier than iron produces a nucleus with lower binding energy per nucleon, which requires energy input (endothermic).
2.3 True.
2.4 False. Iron-56 has the highest (not lowest) binding energy per nucleon (~8.8 MeV), making it the most stable nucleus, not the least stable.
2.5 True.
2.6 False. GW170817 confirmed that neutron star mergers are a major site of r-process nucleosynthesis, not the s-process. The s-process occurs in AGB stars at far lower neutron fluxes.
Q3 — Cloze paragraph
In order: Big Bang / 25 / binding energy / iron-56 / binding energy / endothermic / absorbed / neutron capture / beta decay.
Q4.1 — Why BBN stopped after ~3 minutes
As the universe expanded it cooled rapidly. After approximately three minutes the temperature dropped below ~109 K and the density fell too low for further nuclear fusion to occur. The Coulomb barrier between heavier nuclei could no longer be overcome, halting the process at lithium.
Q4.2 — Why fusion beyond iron requires energy
Nuclei beyond iron-56 lie on the right-hand side of the binding energy per nucleon curve, where successive fusion products have lower binding energy per nucleon than the reactants. Moving away from the peak requires energy input (endothermic), so fusion cannot sustain itself beyond iron.
Q4.3 — r-process vs s-process
The r-process occurs at extremely high neutron flux (supernovae, neutron star mergers): nuclei absorb many neutrons faster than they can beta-decay, creating very neutron-rich unstable isotopes far from the valley of stability. The s-process occurs at low neutron flux (AGB stars): one neutron is captured at a time, with sufficient time for beta decay before the next capture, so intermediate nuclei remain close to stability.
Q4.4 — Mass defect and energy
The mass defect (Δm) is the difference between the total mass of the free constituent protons and neutrons and the actual (smaller) mass of the assembled nucleus. This “missing” mass was converted to binding energy when the nucleus formed, according to $E = \Delta m c^2$. A larger mass defect per nucleon means a more tightly bound, more stable nucleus.
Q5 — Sample concept map
Correct maps should include arrows such as:
- Big Bang — produced most of the universe’s → hydrogen and helium
- stellar nucleosynthesis — builds elements up to → iron-56
- binding energy curve — peaks at → iron-56
- binding energy curve — explains why fusion beyond → iron-56 is endothermic
- r-process — occurs in → neutron star merger
- r-process — produces → gold
- neutron star merger — is confirmed site of → r-process
Award 1 mark per valid labelled arrow (minimum 7, maximum 7 marked).