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• What are the proportions of the main gases in Earth's current atmosphere?

  • Nitrogen: ~4/5 (80%)
  • Oxygen: ~1/5 (20%)
  • Small proportions of other gases such as CO₂ and noble gases
• What percentage of Earth's atmosphere is nitrogen?

  80%
• What percentage of Earth's atmosphere is oxygen?

  20%
• What are the other gases present in Earth's atmosphere and their characteristics?

  • Carbon dioxide (CO₂): Contributes to greenhouse effect
  • Water vapor (H₂O(g)): Influences climate and precipitation
  • Noble gases: Largely inert and unreactive
• What does the symbol ∼ mean?

  Approximately
• Why is evidence about Earth's early atmosphere limited?

  • Time scale of 4.6 billion years
  • Theories have changed over time
  • Limited geological records
• What is one theory about the formation of Earth's early atmosphere?

  During the first billion years of Earth's existence:

  • Intense volcanic activity released gases
  • Early atmosphere was mainly CO₂ with little or no O₂(g)
  • Nitrogen (N₂) gradually built up
  • Small proportions of methane (CH₄) and ammonia (NH₃) were present
• How did oceans form on early Earth?

  Water vapor condensed to form the oceans.
• How did CO₂ levels decrease in the early atmosphere?

  • CO₂ dissolved in water
  • Carbonates precipitated, forming sediments
  • Reduced the amount of CO₂ in the atmosphere
• What process increased oxygen levels in Earth's atmosphere?

  Photosynthesis by algae and plants.
• Write the balanced chemical equation for photosynthesis.

  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• When did algae first start producing oxygen in Earth's atmosphere?

  About 2.7 billion years ago
• Over the next billion years, how did plants affect oxygen levels in the atmosphere?

  The percentage of oxygen gradually increased to a level that enabled animals to evolve.
• How did CO₂ levels decrease in Earth's atmosphere?

  • Photosynthesis by algae and plants
  • Formation of sedimentary rocks containing carbon (e.g., limestone and coal)
  • Production of fossil fuels from decaying plant and animal remains