Carbon-12 and carbon-14 are both isotopes of carbon. Explain why carbon-12 and carbon-14 have the same chemical properties but different physical properties.

Cambridge International IGCSE Chemistry (0620) — 2.3 Isotopes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both isotopes have the same number of protons (6), giving the same atomic number. Chemical properties depend on the number and arrangement of electrons, which are identical for both isotopes, so they behave the same chemically. The isotopes differ in the number of neutrons – carbon‑12 has 6 neutrons, carbon‑14 has 8 neutrons – giving different masses and relative atomic masses. The different masses lead to different physical properties such as density and melting point.

Examiner tips

  • State that identical proton number gives identical electron configuration, hence same chemistry.
  • Mention the different neutron numbers and explain how this changes mass and physical properties.

Common mistakes

  • Confusing the role of electrons with neutrons in determining chemical behaviour.
  • Failing to specify the exact neutron numbers for each isotope.

Mark scheme (4 marks)

  1. Both isotopes have the same number of protons / same atomic number (6)
  2. Chemical properties are determined by the number of electrons / same electron arrangement
  3. The isotopes have different numbers of neutrons (carbon-12 has 6 neutrons; carbon-14 has 8 neutrons)
  4. Different numbers of neutrons give different masses / relative atomic masses, causing different physical properties (e.g. different densities / melting points)

Key terms in this question

isotopes · chemical properties · physical properties

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