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
Memory Aid
78-21-1: Nitrogen-Oxygen-Argon (NOA) For atmospheric evolution, remember the sequence: "Volcanoes → Oceans → Algae → Life → Stability" For what removed CO₂: "POLS" — Photosynthesis, Oceans (dissolved), Limestone (carbonate rocks), Sediment (fossil fuels) Quick Check: Give TWO reasons why CO₂ decreased from the early atmosphere to present levels. Show Answer 1. CO₂ dissolved in the oceans and reacted with minerals to form carbonate rocks (limestone). 2. Photosynthesis by cyanobacteria and plants converted CO₂ to glucose and oxygen.