This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
Air composition, greenhouse effect and climate implications appear in most series.
Approximate proportions
Dry air is a mixture, not a compound
It is made up of:
Nitrogen, N2 — about four-fifths of the air, roughly 78–80%
Oxygen, O2 — about one-fifth, roughly 20–21%
Argon and other noble gases — about 1% in total
Carbon dioxide, CO2 — about 0.04% (small but a key greenhouse gas)
Water vapour — a variable amount depending on humidity
These proportions have stayed roughly constant for the last 200 million years; only the trace gases (especially CO2) have shifted noticeably over the last two centuries