Explain why carbon dioxide is classified as a greenhouse gas but diatomic gases such as nitrogen and oxygen are not.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The concentration of carbon dioxide in Earth's atmosphere has increased significantly since industrialisation, contributing to enhanced global warming. Not all atmospheric gases, however, contribute to the greenhouse effect.
Model answer (4 marks)
Carbon dioxide is a greenhouse gas because it can absorb and re‑emit infrared (IR) radiation from the Earth’s surface. CO₂ is a polyatomic molecule; its bending and stretching vibrations have natural frequencies in the IR region, so it can interact with IR photons and change its dipole moment.
In contrast, the main atmospheric gases N₂ and O₂ are diatomic homonuclear molecules. Their vibrational motions are symmetrical and do not change the electric dipole moment, so they cannot absorb IR radiation. Consequently they do not contribute to atmospheric warming and are not considered greenhouse gases.
In contrast, the main atmospheric gases N₂ and O₂ are diatomic homonuclear molecules. Their vibrational motions are symmetrical and do not change the electric dipole moment, so they cannot absorb IR radiation. Consequently they do not contribute to atmospheric warming and are not considered greenhouse gases.
Examiner tips
- Use the word ‘absorb’ and ‘re‑emit’ to show the greenhouse mechanism.
- Mention that CO₂ is polyatomic with IR‑active vibrations.
- Explain the symmetry of N₂/O₂ and lack of dipole change.
- Show that lack of IR absorption means no greenhouse effect.
Common mistakes
- Claiming N₂/O₂ absorb IR because they are gases; they are not IR‑active.
- Forgetting to mention the dipole moment change or symmetry.
- Using vague terms like ‘they are not greenhouse gases’ without explaining why.
Mark scheme (4 marks)
- Greenhouse gases absorb (and re-emit) infrared radiation emitted by the Earth's surface.
- CO₂ (and other greenhouse gases) are polyatomic molecules whose natural vibrational frequencies correspond to frequencies in the infrared region of the electromagnetic spectrum.
- Diatomic homonuclear molecules such as N₂ and O₂ are symmetrical, so their vibrations do not produce a change in electric dipole moment.
- Because N₂ and O₂ cannot absorb infrared radiation, they do not contribute to warming the atmosphere / do not act as greenhouse gases.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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