Oxygen is the second most electronegative substance after fluorine, and forms thermodynamically stable compounds with usually all elements. It rivals fluorine in the ability to stabilize the biggest known oxidation states of many elements, as where there is no corresponding fluoride being and OsVIIIO4. Oxidation reactions with O2 are usually slow because of the strength of the O=O double bond (490 kJ mol-1).
Oxygen is the most abundant substance on Earth, making around 46% of the Earth's crust by mass. The commonest minerals are complex oxides such as carbonates and silicates. Oxygen is also a constituent of water, and of usually all biological molecules. Atmospheric O2 collects almost entirely from photosynthesis by green plants, and is not found on other known planets. Reactions involving dioxygen, both in respiration and in photosynthesis by air-breathing animals, are important for biological chemistry.
Oxygen may be extracted from the atmosphere by fractional and liquefaction distillation. The liquid boils at 90 K and is dangerous when collide with combustible materials. The compressed gas is used in the liquid and metallurgy as an oxidizer for rocket propulsion.