Oxygen is a chemical element with atomic number of 8 and relative atomic mass of 16.00. In the periodic table of elements, oxygen is one of the oxygen group elements. It is also a highly reactive second period nonmetallic element. It is easy to form compounds (mainly oxides) with almost all other elements. Under standard conditions, two oxygen atoms combine to form oxygen, which is a colorless, odorless and tasteless diatomic gas with the chemical formula of O2. In terms of mass, the content of oxygen in the universe is second only to hydrogen and helium, while in the crust, oxygen is the most abundant element. Oxygen accounts for not only 89% of water mass, but also 20.9% of air volume.