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
greaterelectronegativity.[1] By mass, oxygen is the third-most abundant element
in the universe, after hydrogen and helium[2] and the most abundantelement by
mass in the Earth's crust, making up almost half of the crust's mass.[3] Oxygen
is too chemically reactive to remain a free element inEarth'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.[4] Diatomic oxygen gas constitutes 20.8%
of the volume of air.
