Explain why two atoms that are isotopes of the same element have identical chemical properties but different physical properties such as density.

Eduqas GCSE Physics — 9.1 Nuclear atom and isotopes · Explain · 4 marks · View as Markdown

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

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Model answer (4 marks)

Isotopes have the same number of protons (and electrons), so they have the same electronic structure.
Chemical properties depend on the number and arrangement of electrons, therefore isotopes exhibit identical chemical behaviour.
Isotopes differ in the number of neutrons, giving different mass numbers.
The different masses lead to different physical properties such as density, so physical properties differ.

Examiner tips

  • State that identical proton/electron numbers give the same electronic structure; link to chemical properties.
  • Mention the differing neutron numbers and mass numbers; link to physical properties like density.

Common mistakes

  • Confusing isotopes with ions – emphasise same charge. Failing to mention the role of neutron number in mass. Using vague terms like "similar" instead of "identical" for chemical properties.

Mark scheme (4 marks)

  1. Isotopes have the same number of protons (and electrons), so the same electronic structure
  2. Chemical properties depend on the number / arrangement of electrons, so chemical properties are the same
  3. Isotopes have different numbers of neutrons, giving different mass numbers
  4. Different mass means different physical properties such as density, so physical properties differ

Key terms in this question

isotopes

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