Oxygen makes up about 21% of Earth’s atmosphere by volume.
Most atmospheric oxygen exists as O₂, a diatomic molecule containing two oxygen atoms. It is produced mainly by photosynthesis, especially by plants, algae, and cyanobacteria, which release oxygen while converting light energy into chemical energy.
Oxygen is essential for aerobic respiration in humans and many other organisms. It also supports combustion, although oxygen itself is not a fuel. This distinction explains why a candle burns more vigorously in oxygen but oxygen does not burn independently like hydrogen or gasoline.
Nitrogen is actually the most abundant atmospheric gas, making up about 78% by volume. Argon accounts for about 0.93%, while carbon dioxide is present at a much smaller concentration. Atmospheric percentages can vary slightly with location, altitude, water vapour, and measurement method.