For about 200 million years, the proportions of the gases in the Earth's atmosphere have been much the same as they are today. The atmosphere is a mixture, so the amount of each gas can be given as a percentage of the air.
About four-fifths of the air, approximately 80%, is nitrogen. About one-fifth, approximately 20%, is oxygen. This means there is roughly four times as much nitrogen as oxygen, a ratio of 4 : 1.
The rest is only a small proportion of the air and is made up of various other gases. These include carbon dioxide, water vapour and the noble gases such as argon.