The air around us is a mixture of gases. For about 200 million years the proportions of these gases have been much the same as they are today.
About four-fifths of the atmosphere, approximately 80%, is nitrogen. About one-fifth, approximately 20%, is oxygen. Nitrogen and oxygen together make up nearly all of the air.
The rest is a very small proportion of various other gases. These include carbon dioxide, water vapour and the noble gases such as argon. Carbon dioxide is only a tiny fraction of the air, far less than 1%.
Because nitrogen is about four-fifths and oxygen about one-fifth, there is roughly four times as much nitrogen as oxygen in a sample of air.
