# Explain why the carbon–oxygen bond length in the carbonate ion (CO₃²⁻) is intermediate between a typical C–O single bond and a typical C=O double bond.

> IB DP Chemistry Higher Level (2023 syllabus) — S2.2 The covalent model · Explain · 4 marks

## Mark scheme (4 marks)

1. All three C–O bonds in CO₃²⁻ are equivalent / the electron density is spread equally across all three bonds
2. The bonding is explained by delocalisation of electrons / the pi electrons are delocalised over all three C–O bonds
3. Each C–O bond has a bond order between 1 and 2 (approximately 1.33 / 4/3)
4. Greater bond order than a single bond means greater electron density between nuclei / shorter and stronger than C–O single bond, but less than a full C=O double bond, giving an intermediate bond length

## Key terms

- [bond length](https://www.gradenine.co.uk/glossary/bond-length)
- [carbonate ion](https://www.gradenine.co.uk/glossary/carbonate-ion)

## Related

- [Revision notes for IB DP Chemistry Higher 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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