# 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

## 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

- [Group 17](https://www.gradenine.co.uk/glossary/group-17)
- [Period 3](https://www.gradenine.co.uk/glossary/period-3)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-chlorine-cl-is-placed-3f5c4fc6) · Published by Druglandscape Ltd.