By Doug Leenhouts
Updated Mar 24, 2022
Oxygen is an element that can exist as a solid, liquid, or gas depending on temperature and pressure. In the atmosphere it is found as a gas, specifically a diatomic molecule (O₂). Both the elemental oxygen atoms and oxygen gas are reactive substances essential for life on Earth.
Oxygen gas, or dioxygen, is the second‑most abundant element in Earth's atmosphere, constituting 21% of the air we breathe, after nitrogen at 78%. Pure oxygen has a specific gravity of 1.105, meaning it would sink relative to the rest of the atmosphere if not for wind or convection. ligas.com
Oxygen gas reacts with virtually all elements except the noble gases, forming oxides. For metals such as magnesium, oxidation occurs at standard temperatures and pressures, while heavier elements require elevated temperatures and pressures to induce oxidation. Oxygen is essential for combustion, even though the gas itself is non‑flammable. Many industrial heat‑treating processes rely on bottled oxygen to raise burning temperatures.
Water is approximately 85% oxygen by mass, despite its H₂O formula. The human body is about 60% oxygen, which is why astronomers consider atmospheric oxygen a potential biosignature. Oxides account for roughly 46% of the Earth's crust. In the atmosphere, oxygen exists as diatomic O₂ and as the allotrope ozone (O₃). The ozone layer is only ~3 mm thick, and its depletion is worsened by chlorofluorocarbon emissions.
Oxygen gas is colorless, odorless, and tasteless; ozone and liquid oxygen display a pale blue hue. Ozone’s boiling point is 161.3 K versus O₂’s 90.2 K; its melting point is 80.7 K compared to O₂’s 54.36 K. Ozone is denser (2.144 g L⁻¹) than O₂ (1.429 g L⁻¹). While oxygen is vital for respiration and metabolism, ozone is a potent oxidizer and toxic at high concentrations.
