The Composition of the Atmosphere | Chemistry

📖 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.

📌 Key Note

Atmosphere: ~78% N₂, ~21% O₂, ~0.9% Ar, ~0.04% CO₂. Balance maintained by carbon cycle (photosynthesis/respiration) and nitrogen cycle. Human activities increasing CO₂ — fossil fuels, deforestation. CO₂ small but crucial for photosynthesis and greenhouse effect.

🎯 Matching Activity — Atmospheric Gas — Percentage and Role

Match each gas to its approximate percentage and role. — drag the symbols on the right to match the component names on the left.

Nitrogen (N₂)

Drop here

Oxygen (O₂)

Drop here

Argon (Ar)

Drop here

Carbon dioxide (CO₂)

Drop here

~21% — produced by photosynthesis, consumed by respiration and combustion

~0.04% — greenhouse gas, essential for photosynthesis, rising due to fossil fuels

~78% — unreactive, largest component, cycled by bacteria

~0.9% — noble gas, unreactive, present since Earth formed

⭐ Higher Tier Only

Evaluate methods of measuring atmospheric composition (mass spectrometry, absorption spectroscopy). Consider uncertainty in measurements. Understand that CO₂ at 0.04% is small but critically important for climate and photosynthesis.

🎯 Test Yourself

Question 1 of 2

1. Approximately what percentage of the atmosphere is oxygen?

2. Which process removes CO₂ from the atmosphere?

⭐ How Well Do You Understand This Topic?

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🧪 The Composition of the Atmosphere

Current Composition of the Atmosphere

Maintaining the Balance

Noble Gases and Water Vapour

Mr. Badmus AI