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This lesson covers the composition of the Earth's atmosphere, how it has changed over time, and the evidence for these changes, as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 9. You need to know the approximate composition of today's atmosphere, what the early atmosphere was like, and the processes that caused it to change.
The Earth's atmosphere today has a very stable composition. The main gases are:
| Gas | Approximate percentage |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Argon (Ar) | ~0.9% |
| Carbon dioxide (CO₂) | ~0.04% |
| Water vapour (H₂O) | Variable (0–4%) |
| Other trace gases | Very small amounts (neon, helium, methane, etc.) |
Exam Tip: You must know the approximate percentages of nitrogen (~78%), oxygen (~21%) and carbon dioxide (~0.04%). These numbers come up very frequently in exam questions.
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