The composition of today's atmosphere
Earth's atmosphere has had roughly the same composition for the last 200 million years. The main components and their percentages by volume are:
| Gas | Percentage |
|---|---|
| Nitrogen N₂ | 78 % (about 4/5) |
| Oxygen O₂ | 21 % (about 1/5) |
| Argon Ar | ~1 % |
| Carbon dioxide CO₂ | ~0.04 % (and rising) |
| Water vapour H₂O | variable (0–4 %) |
| Trace gases (other noble gases, methane, ozone) | small |
A handy approximation: 80% N₂, 20% O₂, with everything else under 1%.
Why these percentages?
- Nitrogen is unreactive — once formed, it stays.
- Oxygen comes from photosynthesis by ancient organisms (C9.2).
- Argon is a noble gas — chemically inert, accumulates over geological time.
- CO₂ is recycled rapidly through photosynthesis, respiration, ocean dissolution and combustion.
- Water vapour cycles continuously through evaporation/precipitation.
Daily evidence
- The fact that fires need oxygen but go out under nitrogen confirms ~21% O₂.
- The respiration of plants and animals continuously cycles CO₂ and O₂.
- Greenhouse effect of trace CO₂ has a huge impact despite the small percentage (C9.3).
Some misleading sometimes-true claims
- "Air is 78% nitrogen" → by volume, yes.
- "Air is 78% nitrogen by mass" → NO, oxygen has a higher M_r so contributes ~23% by mass.
- "Most of the atmosphere is oxygen" → no, only 21%.
Trace and variable gases
- Water vapour: 0% in dry desert; up to ~4% in tropical air.
- CO₂: rising from ~280 ppm pre-industrial to ~420 ppm now (still <0.05% but climatically significant).
- Methane CH₄: very low concentration but a powerful greenhouse gas.
⚠Common mistakes
- Mixing up N and O percentages — N₂ is the bigger one.
- Saying water vapour is constant — it's variable.
- Assuming CO₂ is around 5% — it's only ~0.04% (but rising).
- Forgetting argon at ~1% — second most abundant inert gas after N₂.
Links
Sets up C9.2 (evolution of the atmosphere), C9.3 (greenhouse gases), C9.5 (atmospheric pollutants).
AI-generated · claude-opus-4-7 · v3-deep-chemistry