Define the term 'relative atomic mass' and explain why the relative atomic mass of chlorine is not a whole number.

Cambridge International IGCSE Chemistry (0620) — 3.2 Relative masses of atoms and molecules · Define and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Relative atomic mass is the average mass of one atom of an element, expressed in atomic mass units (u) and compared to 1/12 of the mass of a carbon‑12 atom.

The relative atomic mass of chlorine is not a whole number because chlorine occurs as two naturally occurring isotopes,

• chlorine‑35 (≈34.9689 u) and
• chlorine‑37 (≈36.9659 u).

The measured relative atomic mass is a weighted average of these two masses, using their natural abundances (≈75.8 % 35Cl and 24.2 % 37Cl), which gives a value of about 35.45 u – a non‑whole number.

Examiner tips

  • Define the term first, then give the comparison to C‑12; include the unit (u).
  • Explain the isotope mixture and weighted average; give the approximate value to show understanding.

Common mistakes

  • Using the wrong scale (e.g., hydrogen instead of C‑12).
  • Failing to mention that the value is a weighted average of isotopes.
  • Giving the exact isotope masses without explaining the averaging process.

Mark scheme (4 marks)

  1. Relative atomic mass is the average mass of one atom of an element
  2. compared to 1/12 of the mass of one atom of carbon-12 (or compared to one atom of hydrogen / relative to the carbon-12 scale)
  3. Chlorine exists as more than one isotope (chlorine-35 and chlorine-37)
  4. The relative atomic mass is a weighted average of the masses of all naturally occurring isotopes (in their natural abundances), giving a non-whole-number value

Key terms in this question

relative atomic mass

Related

More Relative masses of atoms and molecules questions

▶ Try answering this question with AI marking (free) →