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What is the difference between a diatomic molecule and a polyatomic molecule?

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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.

The primary difference between a diatomic molecule and a polyatomic molecule lies in the number of atoms they contain.

Diatomic molecules consist of exactly two atoms that are chemically bonded together. These can be homonuclear, where both atoms are of the same element (such as oxygen, $O_{2}$ ), or heteronuclear, where the atoms are different (such as carbon monoxide, $C O$ ).12
Polyatomic molecules are defined as molecules containing more than two atoms. Examples include carbon dioxide ( $C O_{2}$ ) and water ( $H_{2} O$ ), though some polymers can consist of thousands of atoms.2

While both types are held together by chemical bonds, diatomic molecules are always bonded in a straight line, whereas polyatomic molecules can adopt more complex, rigid structures with specific bond angles and lengths.12