AQA GCSE Chemistry Topic 9: Chemistry of the Atmosphere

Proportions of Gases in the Atmosphere

• ~80% nitrogen
• ~20% oxygen
• Small amounts of CO2, H2O(g), and noble gases
• ∼ indicates approximately.

Early Atmosphere

• Limited evidence due to the 4.6 billion years timescale.
• Theory of early atmosphere:
    - Formed from volcanic activity releasing gases.
    - Initially may resemble Mars and Venus: mostly CO2, little or no O2.
    - Nitrogen (N2) built up from volcanic activity; small amounts of methane (CH4) and ammonia (NH3).   - Water vapor condensed to form oceans, CO2 dissolved in water, leading to sediment formation and reduced CO2 levels.

Oxygen Increase

• Algae and plants produce oxygen (O2) via photosynthesis:
    6CO_2 + 6H_2O ightarrow C_6H_{12}O_6 + 6O_2
• Algae first produced O2 about 2.7 billion years ago; oxygen levels gradually increased, allowing animal evolution.

Carbon Dioxide Decrease

• Decreased through photosynthesis by algae and plants.
• Formation of sedimentary rocks (e.g., limestone, coal) and fossil fuels from decayed plant and animal matter also contributed to reduced CO2 levels.