Explain why chlorine (Cl) is placed in Group 17 and Period 3 of the periodic table.

IB DP Chemistry Standard Level (2023 syllabus) — S3.1 The periodic table: classification of elements · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Chlorine has 7 electrons in its outermost shell, so it is in Group 17 (the halogens). The group number is equal to the number of valence electrons (or the highest occupied energy level). Chlorine has electrons in three shells (n = 1, 2, 3), so it is in Period 3. The period number is the number of occupied electron shells (or the highest principal quantum number).

Examiner tips

  • State the valence electron count and link it to Group 17.
  • Mention the three occupied shells to justify Period 3.
  • Use the exact wording ‘outermost shell’ and ‘highest principal quantum number’.
  • Keep the answer concise – 4 marks only.

Mark scheme (4 marks)

  1. Chlorine has 7 electrons in its outer/valence shell, which determines its Group 17 placement.
  2. The group number corresponds to the number of electrons in the outer/valence shell (or highest occupied energy level).
  3. Chlorine has 3 occupied electron shells/energy levels, which determines its Period 3 placement.
  4. The period number corresponds to the number of occupied electron shells/energy levels (or the highest principal quantum number occupied).

Key terms in this question

Group 17 · Period 3

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