Today's air: about four-fifths nitrogen and one-fifth oxygen
Definition
Nitrogen
The gas that makes up about four-fifths, or approximately 80%, of Earth's atmosphere.
Definition
Oxygen
The gas that makes up about one-fifth, or approximately 20%, of Earth's atmosphere.
- The atmosphere is the thin layer of gases held around the planet by gravity.
- Nitrogen, N2\text{N}_2N2, is the most abundant gas, making up about four-fifths of the air, which is roughly 80%.
- Oxygen, O2\text{O}_2O2, is the next most abundant, making up about one-fifth, which is roughly 20%.
- Together, nitrogen and oxygen account for about 99% of every sample of clean, dry air.
- Nitrogen is about four times as plentiful as oxygen, so roughly four in every five particles of air are nitrogen molecules.
Key Idea
Picture 100 particles of dry air: close to 80 are nitrogen and close to 20 are oxygen, leaving only a tiny remainder for every other gas.
The small remainder: carbon dioxide, water vapour and noble gases
Definition
Carbon dioxide
A gas made from carbon and oxygen, with one carbon atom for every two oxygen atoms.
Definition
Water vapour
Water in the gaseous state, present in a small proportion of Earth's atmosphere.
- The gases that make up the rest of the air are each present in only small proportions.
- Carbon dioxide, CO2\text{CO}_2CO2, is present at well under 1%, yet it has effects far larger than its tiny amount suggests.
- Water vapour, H2O\text{H}_2\text{O}H2O, is present in a variable amount rather than a fixed one.
- Its proportion is higher over warm oceans and lower in cold, dry air.
- The noble gases of Group 0 make up most of what is left, and the main one is argon, Ar\text{Ar}Ar.
Common Mistake
- Do not write that air is only nitrogen and oxygen.
- The 80% and 20% figures are rounded, so they hide the carbon dioxide, water vapour and noble gases that are still present.
Reading the figures: say 'about', not exact
- The values 80% and 20% are rounded, so pair them with the word about or approximately.
- The proportion of carbon dioxide is small but not zero, so it should never be rounded away to nothing.
- Water vapour is missing from the standard figures because its amount changes from place to place and hour to hour.
Exam technique
- Always tie the figures to the word about or approximately; a bare '80% nitrogen' can lose the mark because the value is only approximate.
- When you name the minor gases, be specific: carbon dioxide, water vapour and a noble gas such as argon, not a vague 'other gases'.
A composition that has held steady for 200 million years
- For about 200 million years, the proportions of the main gases in the air have stayed much the same as they are today.
- That means roughly 80% nitrogen, 20% oxygen and small proportions of other gases across the whole of that time.
- This long stability is why today's figures can be trusted when answering questions about the modern atmosphere.
Self review
- About what fraction, and what percentage, of the air is nitrogen?
- About what fraction, and what percentage, of the air is oxygen?
- Name three gases found in only small proportions in the air.
- Which noble gas makes up most of the small remainder of the air?
- Why is water vapour left out of the fixed percentage figures for the atmosphere?
