Copper has atomic number 29. One isotope of copper has mass number 63 and another has mass number 65. Explain why these two isotopes have the same chemical properties but different mass numbers.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · 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 29 protons, so they have the same atomic number and the same number of electrons (29), giving identical electronic structures. Chemical properties are determined by the arrangement of electrons, so the two isotopes behave the same chemically. The difference in mass number comes from a different number of neutrons: copper‑63 has 34 neutrons and copper‑65 has 36 neutrons, giving the different masses.

Examiner tips

  • Show that both have 29 protons and 29 electrons to justify identical chemistry.
  • Explain that chemical behaviour depends on electron arrangement, not mass.
  • Mention the differing neutron numbers to account for the mass difference.

Common mistakes

  • Confusing mass number with atomic number; stating that the isotopes have different numbers of protons.
  • Over‑explaining the role of neutrons in chemistry; ignoring that neutrons do not affect electronic structure.

Mark scheme (4 marks)

  1. Both isotopes have the same number of protons (29) / same atomic number
  2. Both isotopes have the same number of electrons (29) / same electron arrangement / same electronic structure
  3. Chemical properties depend on the number / arrangement of electrons
  4. The isotopes have different numbers of neutrons (copper-63 has 34 neutrons; copper-65 has 36 neutrons), giving different mass numbers

Key terms in this question

isotope · atomic number · mass number

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