This topic summary covers Knowledge Organiser: Atmosphere Composition within Composition of Atmosphere for GCSE Chemistry. Revise Composition of Atmosphere in Atmosphere for GCSE Chemistry with 20 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Atmosphere Composition
Current Atmosphere
- 78% Nitrogen (N₂)
- 21% Oxygen (O₂)
- ~1% Argon (Ar)
- 0.04% Carbon dioxide (CO₂)
Early Atmosphere
- Mainly CO₂ and water vapour
- Some N₂, CH₄, NH₃
- Little/no oxygen
- Like Venus and Mars today
How CO₂ Decreased
- Dissolved in oceans
- Formed carbonate rocks (limestone)
- Photosynthesis by cyanobacteria
- Buried as fossil fuels
How O₂ Increased
- Cyanobacteria evolved ~3.5 billion years ago
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Plants continued the process
- Now balanced by respiration
Key Equations
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis — removed CO₂, produced O₂)
- CaCO₃ → CaO + CO₂ (thermal decomposition of limestone — used in cement)
- Current atmosphere: ~78% N₂, ~21% O₂, ~1% Ar, ~0.04% CO₂
Common Mistakes
- Saying the early atmosphere contained lots of oxygen: The early atmosphere had little or NO oxygen — O₂ only built up after cyanobacteria evolved and carried out photosynthesis for billions of years
- Forgetting nitrogen is 78% of the atmosphere: Nitrogen is the most abundant gas — students often say oxygen is the main gas, but O₂ is only ~21%
- Saying CO₂ only decreased because of photosynthesis: CO₂ decreased due to photosynthesis, but also dissolved in oceans AND formed carbonate rocks — give all three routes for full marks
- Confusing early atmosphere with current atmosphere: Early atmosphere = mainly CO₂; current atmosphere = mainly N₂ — the change took billions of years
Practice questions for Composition of Atmosphere
What is the approximate percentage of nitrogen in the current atmosphere?
Describe how Earth's early atmosphere was formed.
Quick recall flashcards
How was oxygen added to the atmosphere?
Photosynthesis by plants and algae
What percentage of the atmosphere is CO₂?
About 0.04%