A colorless, odorless, tasteless, gaseous chemical element that occurs free in the atmosphere, forming one fifth of its volume, and in combination with water, sandstone, limestone, etc. It is very active, combines with nearly all other elements, is the most common element in the earth's crust, and is essential to life processes and to combustion.

Oxygen was discovered in 1774 by Joseph Priestley in Wiltshire, England by heating mercuric oxide, and independently by Carl Wilhelm Scheele in Upssala, Sweden in 1772, although he did not publish his findings until after Priestley. Priestley originally named the gas "dephlogisticated air" but Antoine Laurent Lavoisier subsequently refuted phlogiston theory and named it oxygen (Greek oxys, acid + Latin gignere, to beget) in 1777, because it was believed that oxygen is present in all acids.

Symbol: O
Atomic number: 8
Atomic weight: 15.9994
Density (at 0°C with 101,325 pascals): 1.429 g/L
Melting point: -218.4°C
Boiling point: -182.96°C
Valence: -2, +2
Ground state electron configuration: [He]2s22p4