The proportions of different gases in the atmosphere: small proportions of various other gases, including carbon dioxide, water vapour and noble gases.

The idea

The atmosphere is a mixture, not a compound, so each gas keeps its own properties and can be separated by physical means. About four-fifths of the air is nitrogen and about one-fifth is oxygen, leaving only small proportions of other gases. These trace gases include carbon dioxide, water vapour and the noble gases, mainly argon. Carbon dioxide is released by combustion and respiration and is used by photosynthesis, so its concentration varies slightly with location, season and time of day. Water vapour varies greatly, condensing to form clouds and rain. Noble gases such as argon are chemically inert because they have full outer electron shells, so they are used in lamps and as shielding gases. A useful method is to treat dry air as roughly 78% nitrogen, 21% oxygen and about 1% argon plus other trace gases, then explain why the percentages are approximate rather than fixed.

What earns the marks

• State that nitrogen makes up about four-fifths (roughly 78%) of the atmosphere and oxygen about one-fifth (roughly 21%).
• Identify carbon dioxide, water vapour and noble gases as present in small proportions, with argon as the most common noble gas.
• Explain that the atmosphere is a mixture of gases, so the proportions can be described as percentages by volume.
• Recognise that carbon dioxide and water vapour concentrations vary with location, weather, season and human activity, so quoted values are approximate.
• Link the chemical inertness of noble gases to their full outer electron shells and their uses, such as argon in filament lamps.

Common mistakes and how to put them right

• Writing that oxygen is the most abundant gas in the atmosphere.

  Put it right: Nitrogen is most abundant at about four-fifths, with oxygen at about one-fifth.

• Treating carbon dioxide as a major component rather than a trace gas.

  Put it right: It is present in small, variable proportions, even though it is important for climate and photosynthesis.

• Describing air as a compound of nitrogen and oxygen.

  Put it right: Air is a mixture, so the gases are not chemically bonded and can be separated by fractional distillation.

Exam tips

• Quote approximate proportions as fractions or percentages, for example nitrogen about four-fifths and oxygen about one-fifth, and make clear they are approximate.
• When asked to name trace gases, give carbon dioxide, water vapour and a named noble gas such as argon rather than saying only 'other gases'.
• Link any question about varying carbon dioxide or water vapour to a process such as combustion, respiration, photosynthesis or condensation to gain explanation credit.