oxygen

The second most abundant component of the gaseous mixture that makes up the atmosphere. It occupies 20.95% of its volume, being present in the air mostly in its diatomic form (O₂). Oxygen molecules preferentially dissociate in the upper atmosphere, giving rise to free atoms of that element. Like nitrogen, it is a colorless, odorless, and tasteless gas, fundamental for life, which living beings consume through respiration. The photosynthetic activity of plants and other organisms such as cyanobacteria and marine phytoplankton is the mechanism responsible for replenishing it, incorporating it into the air.