The proportions of different gases in the atmosphere: about one-fifth (approximately 20%) oxygen

The idea

Oxygen is the second most abundant gas in the modern atmosphere, making up about one-fifth of dry air, which is approximately 20%. Like the other atmospheric proportions, this is a proportion by volume, and for gases it is effectively the same as the proportion by number of molecules. A helpful method is to imagine 100 molecules of dry air: about 20 are oxygen and about 80 are nitrogen, with the remaining small fraction made up of argon, carbon dioxide and other trace gases. Oxygen is the gas that supports combustion and is used in respiration by most living organisms, so its proportion matters for life on Earth. When describing the atmosphere, always pair the gas with its approximate proportion and keep the values for nitrogen and oxygen the correct way round.

What earns the marks

• States that oxygen makes up about one-fifth, or approximately 20%, of the atmosphere.
• Recognises that the proportion is a fraction or percentage of the total volume of dry air.
• Links the value to a simple model, such as 20 out of every 100 molecules being oxygen.
• Distinguishes oxygen from nitrogen, which is the most abundant gas at about four-fifths.
• Uses the correct gas name, oxygen, rather than a formula or a vague description.
• Applies the figure when comparing the abundances of atmospheric gases.

Common mistakes and how to put them right

• Writing that oxygen is about 80% of the atmosphere.

  Put it right: This swaps the values for nitrogen and oxygen, so remember oxygen is the smaller share at about 20%.

• Confusing the proportion of oxygen with the proportion of carbon dioxide, which is a trace gas at roughly 0.04%.

  Put it right: Keep trace gases clearly separate from the main two.

Exam tips

• Give the proportion as both a fraction and a percentage, for example one-fifth or about 20%, to make the value unambiguous.
• When comparing nitrogen and oxygen, state which gas is larger and by how much, for example nitrogen is about four times as abundant as oxygen.
• Read the question carefully to check whether it refers to dry air or to air including water vapour, because the stated percentages change slightly.