# 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

> 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.

## 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

- [anomalous](https://www.gradenine.co.uk/glossary/anomalous)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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