Oxygen / O₂

Dioxygen, commonly referred to as oxygen or molecular oxygen by popularisation, is a gaseous substance under normal temperature and pressure conditions. It was discovered on 1 August 1774 by the English chemist Joseph Priestley, and given the name "oxygen" by the French chemist Antoine Lavoisier in 1778.
Comprising 20.95% of the Earth's atmosphere in terms of volume (23.2% in terms of mass), dioxygen is an essential gas for the majority of current life forms, to which it provides the fuel required for cells to function (cellular respiration). In the aquatic environment, dissolved dioxygen is also required for the survival of a large amount of species. It plays a role in redox reactions, particularly in relation to combustion, corrosion and respiration.
It is a colourless gas at ambient temperature and pressure; liquefied gas: colourless-to-blue liquid.