📖 In-Depth Theory
Current Composition of the Atmosphere
The Earth's atmosphere has been approximately STABLE for the last 200 million years.
APPROXIMATE COMPOSITION BY VOLUME:
NITROGEN (N₂): ~78%
OXYGEN (O₂): ~21%
ARGON (Ar): ~0.9%
CARBON DIOXIDE (CO₂): ~0.04% (rising due to human activity)
Other gases: water vapour (variable), neon, helium, methane etc. — tiny amounts.
Simplified: approximately 4/5 nitrogen and 1/5 oxygen.
The proportions of gases are maintained by:
PHOTOSYNTHESIS — plants absorb CO₂ and release O₂.
RESPIRATION — all living organisms absorb O₂ and release CO₂.
COMBUSTION — burning fuels consumes O₂ and produces CO₂.
NITROGEN CYCLE — bacteria fix and release N₂ from/to the atmosphere.
Maintaining the Balance
The current balance is maintained by natural CYCLES:
CARBON CYCLE:
CO₂ absorbed: photosynthesis (plants), dissolving in oceans.
CO₂ released: respiration, combustion, decomposition, volcanic activity.
OXYGEN BALANCE:
O₂ produced: photosynthesis.
O₂ consumed: respiration, combustion, oxidation of metals.
NITROGEN:
N₂ is extremely unreactive — doesn't participate in most reactions.
Fixed by nitrogen-fixing bacteria in soil (convert N₂ to nitrates for plants).
Released back by denitrifying bacteria.
Human activities are disrupting these balances:
Burning fossil fuels → increases CO₂.
Deforestation → reduces CO₂ absorption and O₂ production.
Industrial processes → release various gases.
Noble Gases and Water Vapour
ARGON (Ar) — 0.9%:
A noble gas — extremely unreactive, single atoms (monatomic).
Present in the atmosphere since the Earth formed — doesn't participate in any reactions.
Uses: filling light bulbs, welding (inert atmosphere prevents oxidation).
WATER VAPOUR:
Variable amount — ranges from near 0% in deserts to ~4% in humid tropical air.
Evaporates from oceans, lakes, rivers.
Condenses to form clouds and rain.
Plays a crucial role in weather and climate.
CARBON DIOXIDE — ~0.04%:
Small but critically important:
ESSENTIAL for photosynthesis — all plant life depends on it.
GREENHOUSE GAS — absorbs infrared radiation and warms the Earth.
DISSERVES IN OCEANS — oceans are a major carbon sink.
RISING due to fossil fuel combustion.
⚠️ Common Mistake
The atmosphere is approximately 78% NITROGEN and 21% OXYGEN — not the other way round. Many students think oxygen is the main component because we need it to breathe — but nitrogen is far more abundant. CO₂ is only about 0.04% — tiny but critically important for the greenhouse effect and photosynthesis.