What is in the air today?
The atmosphere is the layer of gas around Earth. For about the last 200 million years its make-up has stayed nearly the same.
- Nitrogen (N2): about 78% (roughly four-fifths).
- Oxygen (O2): about 21% (roughly one-fifth).
- Argon: about 0.9%.
- Carbon dioxide (CO2): about 0.04%.
- Tiny amounts of neon, helium, methane and others. Water vapour changes from place to place.
These numbers are for dry air. A quick check: nitrogen + oxygen ≈ 99%.
Earth's early atmosphere
Earth is about 4.6 billion years old. In its first billion years there was a lot of volcanic activity. Volcanoes released gases. Scientists think the early air was mostly carbon dioxide with water vapour, some nitrogen and small amounts of methane and ammonia. There was little or no oxygen.
This is a bit like the air of Mars and Venus today, which is mostly CO2.
Why are we not sure?
Nobody was there to measure it, and the evidence (old rocks, gas bubbles in ice, fossils) is limited. So there are different theories. This is a good example of how science uses evidence and changes its ideas.
How oxygen increased
As Earth cooled, water vapour condensed (turned to liquid) and fell as rain. This formed the oceans.
About 2.7 billion years ago, simple algae (tiny green living things in the sea) began to do photosynthesis:
carbon dioxide + water → glucose + oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
Over the next billion years or so, plants appeared too. Oxygen slowly built up until there was enough for animals to live.
How carbon dioxide decreased
CO2 went down in three main ways:
- Dissolving in oceans: carbon dioxide dissolves in water and forms carbonates.
- Photosynthesis: algae and plants used CO2 to make food.
- Locking carbon in rocks: sea creatures used carbonates to build shells and skeletons. These piled up and became sedimentary rocks such as limestone (calcium carbonate). Buried plants became coal; buried tiny sea creatures became crude oil and natural gas.
Fossil fuels are "stored ancient CO2". Burning them returns that carbon to the air.
Try it: model the air with 100 counters
Take 100 beads, buttons or rice grains. Make 78 one colour (nitrogen), 21 another (oxygen) and 1 a third (argon and others). Put them in a jar. Now make a second jar for early Earth: mostly "CO2" counters. Swap counters one by one and say what process changed each one (dissolving, photosynthesis, locking in rock).
Key formulas and definitions
- Dry air today ≈ 78% N₂, 21% O₂, 0.9% Ar, 0.04% CO₂
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Limestone = calcium carbonate, CaCO₃
- Fossil fuels (coal, oil, gas) = carbon taken from ancient air
Worked examples
1. A classroom holds 200 m³ of air. Roughly what volume is oxygen?
Oxygen is about 21%. 21% of 200 = 0.21 × 200 = 42 m³.
2. Explain why there was little oxygen in the early atmosphere.
Oxygen in today's air was made by photosynthesis. Before algae and plants existed, nothing made oxygen in large amounts, and volcanoes give out CO₂ and water vapour, not oxygen.
3. Give two ways carbon dioxide left the early atmosphere and say where the carbon went.
1) It dissolved in the oceans, then formed carbonates that sea creatures used for shells; the shells became limestone. 2) Plants used it in photosynthesis; buried plants became coal. So the carbon ended up in sedimentary rocks and fossil fuels.
Common mistakes
- Saying air is mostly oxygen. It is mostly nitrogen (78%); oxygen is only about 21%.
- Thinking CO₂ is a large part of air today. It is only about 0.04%.
- Saying oxygen came from volcanoes. Volcanoes gave CO₂ and water vapour; oxygen came from photosynthesis.
- Forgetting the oceans: water vapour condensing to form oceans, and CO₂ dissolving in them, is a key step.