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Why is oxygen written as O2 rather than O?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

Oxygen is written as $O_{2}$ because it exists as a diatomic molecule under normal conditions, rather than as a single, isolated atom ( $O$ ).1

While the atomic species, $O$ , is highly reactive, the molecular form, $O_{2}$ , is more stable at ambient temperatures and pressures. In this diatomic state, oxygen atoms share electrons to achieve a more stable electronic configuration, consistent with the principles of chemical bonding and the octet rule, which describes how elements tend to gain, lose, or share electrons to attain the stable outer-shell configuration of noble gases.12

The transition from the atomic state to the molecular state is energetically favorable, as evidenced by the large energy required to dissociate $O_{2}$ into two oxygen atoms. Consequently, in nature and standard chemical notation, oxygen is represented as $O_{2}$ to reflect its preferred molecular structure.1