A student is investigating the composition of the universe and the Earth's crust. The student learns that the most abundant element in the universe is hydrogen, whilst the most abundant element in the Earth's crust is oxygen. Describe how elements are formed in stars and explain why the composition of the universe differs so greatly from the composition of the Earth's crust.

OCR A-Level Chemistry B: Salters (H433) — EL Elements of life · Describe and Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Hydrogen makes up approximately 75% of all normal matter in the universe by mass. Oxygen makes up approximately 46% of the Earth's crust by mass.

Model answer (5 marks)

1. Hydrogen and helium were produced in the Big Bang and shortly thereafter.
2. In stars, nuclear fusion fuses hydrogen into helium and then heavier elements up to iron.
3. Elements heavier than iron are produced in supernova explosions.
4. The universe remains dominated by hydrogen because most hydrogen has not yet been fused into heavier elements.
5. The Earth's crust is rich in oxygen and heavier elements because it formed from the debris of exploded stars, concentrating these heavier elements in planetary bodies.

Examiner tips

  • Use the word ‘fused’ for nuclear fusion, ‘supernova’ for heavy‑element production; link the Big Bang to the abundance of hydrogen; explain why the crust is richer in heavy elements by referencing stellar debris.
  • Keep each point short and to the point – the scheme rewards concise, accurate statements. Use the exact terminology: ‘Big Bang’, ‘nuclear fusion’, ‘supernova’.
  • Include the causal link: Big Bang → hydrogen dominance; stellar fusion → heavier elements; supernova → heavy elements in crust.
  • Use the correct order: Big Bang → fusion in stars → supernova → planetary composition.

Common mistakes

  • Confusing the Big Bang with stellar fusion; writing that all hydrogen is fused in stars. Failing to mention supernovae as the source of elements heavier than iron. Using vague terms like ‘many elements’ instead of specifying fusion and supernova processes.

Mark scheme (5 marks)

  1. Hydrogen and helium were formed in the Big Bang / shortly after the Big Bang
  2. Heavier / larger elements are formed by nuclear fusion in stars
  3. Elements heavier than iron are formed when stars explode / in supernovae
  4. The universe is mostly hydrogen because hydrogen was the starting material / most abundant element formed in the Big Bang and has not all been converted by fusion
  5. The Earth's crust is rich in oxygen / heavier elements because it is made from the remains of exploded stars / stellar debris, so heavier elements became concentrated in planets

Key terms in this question

elements · universe

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