The proportions of different gases in the atmosphere: about four-fifths (approximately 80%) nitrogen

The idea

The modern atmosphere is a mixture of gases, and nitrogen is by far the most abundant. Roughly four out of every five molecules in dry air are nitrogen, so the proportion is about 80% or four-fifths. This value is a proportion by volume, which for gases is effectively the same as proportion by number of molecules. A useful method is to picture 100 gas molecules chosen at random from dry air: about 80 would be nitrogen, about 20 oxygen, and only a small number would be argon, carbon dioxide, water vapour and other trace gases. Nitrogen is unreactive under normal conditions, which helps explain why its proportion has remained stable for a very long time. When asked to describe the atmosphere, state the gas and its approximate proportion, and remember that the figures refer to dry air unless water vapour is mentioned.

What earns the marks

• States that nitrogen makes up about four-fifths, or approximately 80%, 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 80 out of every 100 molecules being nitrogen.
• Distinguishes nitrogen from oxygen, which is the second most abundant gas at about one-fifth.
• Uses the correct gas name, nitrogen, rather than referring vaguely to 'air' or 'the main gas'.
• Applies the figure when comparing the abundances of atmospheric gases.

Common mistakes and how to put them right

• Writing that nitrogen is about 20% of the atmosphere.

  Put it right: This confuses nitrogen with oxygen, so learn the pair as nitrogen 80% and oxygen 20%.

• Describing nitrogen as a compound or as a reactive gas.

  Put it right: Nitrogen is an element and is relatively unreactive, which is why its proportion is stable.

Exam tips

• Quote the proportion as a fraction and a percentage, for example four-fifths or about 80%, to show the value clearly.
• If a question asks you to compare gases, give both the gas name and its proportion rather than the proportion alone.
• Check whether the question says dry air; if water vapour is included, the percentages of the other gases are slightly lower, so read the wording carefully.