Explain why hydrogen is considered anomalous in the periodic table and cannot be unambiguously placed in a single group.

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).

Hydrogen is the lightest and most abundant element in the universe. Although it is most commonly shown at the top of Group 1 in the periodic table, some versions place it in Group 17, and a few place it separately from all groups.

Model answer (4 marks)

Hydrogen has one valence electron, so it can lose one electron to give H⁺, just like the alkali metals in Group 1. It can also gain one electron to give H⁻, just like the halogens in Group 17. However, hydrogen is a non‑metallic gas at standard conditions and lacks the metallic properties of Group 1 (lustre, malleability, conductivity). Because it shares features with both groups but is not truly either, it is considered anomalous and is often shown separately in the periodic table.

Examiner tips

  • Mention the valence‑electron similarity to both groups first, then the physical‑property difference; keep it concise to fit the 4 marks.
  • Use the exact terms ‘alkali metals’, ‘halogens’, ‘non‑metal’, ‘1+ ion’, ‘1− ion’ to match the mark scheme.

Mark scheme (4 marks)

  1. Hydrogen has one valence electron, which resembles Group 1 (alkali metals), as both can lose one electron to form a 1+ ion.
  2. Hydrogen can also gain one electron to form H⁻ (hydride ion), which resembles Group 17 (halogens) that also gain one electron to form 1− ions.
  3. Unlike Group 1 metals, hydrogen is a non-metal (gas) at standard conditions, so it does not share the physical properties of alkali metals such as metallic lustre, malleability, or electrical conductivity.
  4. Because hydrogen shares properties with both Group 1 and Group 17 yet is identical to neither, it is considered anomalous and is sometimes placed separately, reflecting its unique chemistry.

Key terms in this question

anomalous

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