A science journalist is writing an article about the origins of the chemical elements found in living organisms. The journalist states: 'The atoms in your body were forged in stars billions of years ago.' Explain how nuclear fusion in stars produces heavier elements from hydrogen, and describe how these elements eventually become part of living organisms on Earth.

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

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

All living organisms are made mainly from a small number of non-metal elements. These elements were not present at the very beginning of the universe but were formed later through processes inside stars.

Model answer (5 marks)

1. The Big Bang produced mainly hydrogen (and helium). 2. In stellar cores, high temperature and pressure allow nuclear fusion to combine hydrogen nuclei into helium and, through successive fusion steps, into progressively heavier nuclei. 3. Fusion reactions in stars build elements up to iron, the most tightly bound nucleus. 4. When massive stars exhaust their fuel they explode as supernovae, ejecting the newly formed elements into the interstellar medium. 5. These elements are incorporated into the gas and dust that formed the Solar System; they are now part of the Earth’s crust and oceans and are incorporated into the molecules that make up living organisms.

Examiner tips

  • Use the exact phrase "nuclear fusion joins smaller nuclei together" to gain the fusion point.
  • Mention that fusion stops at iron to show understanding of stellar nucleosynthesis.
  • Explain the supernova release to link stars to Earth.
  • Show the chain from stars to Earth to life to earn all 5 marks.

Common mistakes

  • Confusing fusion with fission; students say elements are made by fission.
  • Forgetting that fusion only produces elements up to iron.
  • Failing to mention the supernova or the dispersal of elements into space.

Mark scheme (5 marks)

  1. Hydrogen was the first element formed / the universe began with hydrogen (and helium)
  2. Nuclear fusion joins smaller nuclei together to form larger/heavier nuclei
  3. Elements up to iron are formed by nuclear fusion inside stars
  4. When stars explode (supernova) / die, the elements are released / dispersed into space
  5. These elements became part of the Earth / were incorporated into molecules that make up living organisms

Key terms in this question

nuclear fusion · element · hydrogen

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