Chlorine has an atomic number of 17 and its most common isotope has a mass number of 35. State the number of protons, neutrons and electrons in this isotope of chlorine. Then explain why the relative atomic mass of chlorine, shown in the periodic table, is approximately 35.5 rather than a whole number.

WJEC A-Level Chemistry (Wales) — 2.3 Transition metals (A-Level only) · State and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Protons: 17
Neutrons: 18
Electrons: 17
The relative atomic mass of chlorine is about 35.5 because chlorine has two stable isotopes, ^35Cl and ^37Cl. The natural abundance of ^35Cl is roughly 75 % and ^37Cl about 25 %. The weighted average of their masses (35 × 0.75 + 37 × 0.25) gives 35.5, so the atomic mass is not a whole number.

Examiner tips

  • State the exact numbers for protons, neutrons and electrons first. Explain the isotope concept before giving the weighted‑average reason. Use the word ‘average’ and ‘abundance’ to match the mark scheme.
  • common_mistakes
  • :
  • Saying chlorine has only one isotope. Giving the wrong neutron number (e.g. 17 instead of 18). Failing to mention the weighted average or natural abundance.

Mark scheme (5 marks)

  1. Number of protons = 17
  2. Number of neutrons = 18 (mass number minus atomic number: 35 − 17)
  3. Number of electrons = 17 (equal to number of protons in a neutral atom)
  4. Chlorine has more than one isotope / chlorine exists as isotopes with different numbers of neutrons (e.g. chlorine-35 and chlorine-37)
  5. Relative atomic mass is an average value that takes account of the abundance of the isotopes, so it is not a whole number

Key terms in this question

isotope · atomic number · mass number · relative atomic mass · neutron

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