# Explain why carbon dioxide is a greenhouse gas but nitrogen (N₂) is not, with reference to molecular structure and the interaction of molecules with infrared radiation.

> IB DP Physics Higher Level (2023 syllabus) — B.2 Greenhouse effect · Explain · 4 marks

## Mark scheme (4 marks)

1. CO₂ has a dipole moment that can change during certain vibrational modes (it is IR-active), whereas N₂ is a homonuclear diatomic molecule and its bond dipole moment cannot change during vibration.
2. The changing dipole moment in CO₂ allows it to absorb photons of infrared radiation whose frequency matches the natural vibrational frequency of the molecule (resonance absorption).
3. N₂ cannot absorb infrared radiation because its symmetric vibration produces no change in dipole moment, so there is no mechanism for the photon's oscillating electric field to couple to the molecule.
4. After absorbing IR radiation, CO₂ molecules re-emit infrared radiation in all directions, including back towards Earth's surface, raising the surface temperature — this is the greenhouse effect.

## Key terms

- [greenhouse gas](https://www.gradenine.co.uk/glossary/greenhouse-gas)
- [infrared radiation](https://www.gradenine.co.uk/glossary/infrared-radiation)

## Related

- [Revision notes for IB DP Physics 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-carbon-dioxide-is-a-81618301) · Published by Druglandscape Ltd.