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By the end of these notes, you should be able to:
The atmosphere is the layer of gases that surrounds the Earth. Think of it like an invisible blanket wrapped around the planet. It is held in place by Earth's gravity (the pulling force that keeps everything on the ground).
The atmosphere is incredibly important because it:
The atmosphere is a mixture of gases. The most important gases and their approximate percentages are:
| Gas | Approximate % of the Atmosphere |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Argon (Ar) | ~0.93% |
| Carbon dioxide (CO₂) | ~0.04% |
| Water vapour (H₂O) | Variable (0–4%) |
Here is a simple description of each gas:
Nitrogen — The most abundant (plentiful) gas in the atmosphere. It makes up nearly four-fifths of the air you breathe. Nitrogen is mostly unreactive, meaning it does not easily combine with other substances.
Oxygen — The second most abundant gas. Oxygen is essential for respiration (the process living things use to release energy from food). It is also needed for burning (combustion).
Argon — A very small but notable portion of the atmosphere. Argon is an inert gas, meaning it is almost completely unreactive — it does not combine with other substances under normal conditions.
Carbon dioxide (CO₂) — Present in a very small amount, but it plays a hugely important role in the greenhouse effect (explained below). It is also used by plants during photosynthesis (the process plants use to make food from sunlight).
Water vapour — The gaseous (invisible) form of water. The amount of water vapour in the air changes depending on where you are and what the weather is like — it is higher above oceans and lower in deserts. Water vapour also plays an important role in the greenhouse effect.
💡 Remember: Nitrogen and oxygen together make up about 99% of the atmosphere. All the other gases — including argon, carbon dioxide, and water vapour — make up only about 1%.
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