Oxygen is a chemical element with symbol O and atomic number 8. Its name derives from the Greek roots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γόνος (-gοnos) ("producer", literally "begetter"), because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula O2. This substance is an important part of the atmosphere, and is necessary to sustain most terrestrial life.
Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with most elements except the noble gases Helium and Neon. Oxygen is a strong oxidizing agent and only fluorine has greater electronegativity. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass. Oxygen is too chemically reactive to remain a free element in Earth's atmosphere without being continuously replenished by the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Free elemental O2 only began to accumulate in the atmosphere about 2.5 billion years ago (see Great oxygenation event) about a billion years after the first appearance of these organisms. Diatomic oxygen gas constitutes 20.8% of the volume of air.