The atmosphere consists mainly of nitrogen and oxygen, with smaller proportions of other gases such as carbon dioxide. The amount of carbon dioxide in the atmosphere is maintained through a balance between processes such as photosynthesis, respiration and combustion. But human activities are polluting the atmosphere.
Photosynthesis by plants is thought to be a key process in the evolution of the Earth’s atmosphere.
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.
It is clear that the main gas is nitrogen at 78 per cent. Oxygen is the next most abundant gas at 21 per cent. This is the gas that allows animals and plants to respire [respire: To engage in respiration - the energy-producing process inside living cells ], and fuels to burn. These two gases are both elements and account for about 99 per cent of the gases in the atmosphere.
The remaining gases are found in much smaller proportions. These include carbon dioxide (0.035 per cent), water vapour and noble gases such as argon.
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