Oxygen

Oxygen is a reactive nonmetal that readily combines with many other elements to form compounds called oxides. At room temperature, its familiar form is a colorless, odorless gas made of pairs of oxygen atoms, O₂. It makes up about 21% of dry air by volume and helps our cells release energy from food. Plants, algae, and cyanobacteria replenish atmospheric oxygen through photosynthesis. Oxygen atoms are also chemically bound in water and in many minerals in Earth's crust. Industry obtains oxygen mainly by separating it from air: cooling air into a liquid allows its components to be separated by their different boiling points. Oxygen is used in medical breathing support and in steelmaking, where it reacts with excess carbon in molten iron.