Oxygen is the eighth element on the periodic table and as such has eight protons in its nucleus. Oxygen is produced in the cores of stars through the fusion of more simple elements like Hydrogen and Helium. Most oxygen atoms (>99.7%) have eight neutrons in their nucleus and isotopes are relatively rare.

Due to its small atomic number and stability, there are many signatures of oxygen in the Universe. At optical wavelengths the forbidden oxygen III emission line is prevalent in many galactic spectra, whilst at radio wavelengths the OH megamasers are visible at great distances from the Sun in external galaxies.

At standard temperatures and pressures on Earth, pure oxygen forms the diatomic molecule O₂. On Earth oxygen is essential for life. We breathe in O₂ and expel CO₂. Water is another example of a compound (H₂O) that also contains oxygen. Oxygen is a very reactive element and quite corrosive.