Explain why atoms of the same element can have different masses, and outline how this affects the relative atomic mass (Ar) value shown on the periodic table.

IB DP Chemistry Standard Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Atoms of the same element have the same number of protons but different numbers of neutrons, so they are isotopes. Isotopes have different mass numbers and therefore different masses. The relative atomic mass (Ar) shown on the periodic table is a weighted mean of the masses of all naturally occurring isotopes, weighted by their relative abundances. Because the isotopes occur in different proportions, the Ar value is usually not a whole number.

Examiner tips

  • Use the term "isotopes" and mention different neutron numbers
  • Explain weighted mean and relative abundances
  • Show why Ar is not an integer
  • Keep answer concise and to the point

Common mistakes

  • Confusing atomic number with mass number
  • Saying all atoms have the same mass
  • Forgetting to mention weighted mean or relative abundances

Mark scheme (4 marks)

  1. Atoms of the same element have the same number of protons (same atomic number) but a different number of neutrons.
  2. These atoms are called isotopes; they have different mass numbers (nucleon numbers) and therefore different masses.
  3. The relative atomic mass is a weighted mean / average of the masses of all naturally occurring isotopes of the element.
  4. Because isotopes occur in different proportions (relative abundances), the Ar value is rarely a whole number.

Key terms in this question

relative atomic mass

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