Hydrogen has three isotopes: protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3). Explain why all three isotopes of hydrogen have identical chemical properties but different physical properties such as density.

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)

All three isotopes have the same number of protons (1) and therefore the same atomic number.
They have identical chemical properties because each has the same number of electrons (1) and the same electronic configuration.
The isotopes differ in the number of neutrons: protium 0, deuterium 1, tritium 2.
The different neutron numbers give different masses and relative atomic masses, which in turn give different physical properties such as density.

Examiner tips

  • Mention atomic number and electronic configuration for chemical similarity. State neutron differences for physical differences. Keep answer concise to fit 4 marks.

Common mistakes

  • Confusing isotopes with elements. Forgetting to mention neutrons. Using vague terms like "different" without specifying mass or density.

Mark scheme (4 marks)

  1. All three isotopes have the same number of protons / the same atomic number (1 proton each)
  2. Identical chemical properties because all three have the same number of electrons / same electronic configuration
  3. The isotopes have different numbers of neutrons (protium has 0, deuterium has 1, tritium has 2)
  4. Different numbers of neutrons give different masses / different relative atomic masses, leading to different physical properties such as density

Key terms in this question

isotopes

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