What is the most abundant gas in Earth's atmosphere?

Dry air is about 78 per cent nitrogen by volume, with oxygen making up roughly 21 per cent and argon most of the small remainder. Nitrogen dominates largely because it is so unreactive. Its two atoms are locked together by a triple bond that takes a great deal of energy to break, so nitrogen released by volcanoes and by biological processes has simply accumulated over geological time instead of being consumed. Oxygen, by contrast, reacts eagerly with rock, metal and living tissue, and its high level in our air is actively maintained by photosynthesis. If plants and algae stopped, oxygen would slowly be drawn down by weathering and respiration. The inertness of nitrogen is also useful: it is pumped into food packaging and fuel tanks precisely because it will not support combustion. Water vapour is left out of the standard figures because its share swings from almost nothing in a desert to several per cent in the humid tropics.

Bacteria that convert atmospheric nitrogen into usable compounds make nearly all life on land possible.