2O2

2O2 denotes two molecules of dioxygen (O2). It is not a distinct chemical substance; rather, it is a notation used in chemical equations to indicate an amount of oxygen gas. In this sense, 2O2 is a stoichiometric quantity rather than the name of a compound.

In stoichiometry, coefficients indicate molar amounts. Therefore, 2O2 typically means 2 moles of O2 when the equation is written in molar terms, corresponding to 4 oxygen atoms in total. The mass corresponding to 2 moles of O2 is 64.00 grams (since one mole of O2 has a mass of 32.00 g).

Oxygen gas (O2) is a colorless, odorless diatomic molecule that makes up about 21% of Earth's atmosphere. It has a double bond between the two oxygen atoms and is paramagnetic due to unpaired electrons in its excited state. O2 is essential for aerobic respiration in most organisms and is also a key reactant in combustion and various industrial processes.

There is also a weakly bound van der Waals dimer of two O2 molecules, often written (O2)2 and sometimes referred to as O4 in informal contexts. This species exists only transiently under specific conditions and is not a stable, separate molecule under standard laboratory conditions.

In practical terms, the notation 2O2 appears in balanced chemical equations to show that two molecules (or two moles) of oxygen gas participate as reactants or are produced as products. A common example is the combustion of methane: CH4 + 2 O2 -> CO2 + 2 H2O, where 2 O2 indicates two molecules of O2 are consumed.