The bonds in the oxide of boron are covalent, with electron-pair bonds between B and O. Because each boron atom has three outer-shell electrons for sharing, whereas each oxygen atom needs two electrons to share in pair bonds, the ratio of B to O atoms must be 2:3, just as if the interacting species were and ions.

Although the overall composition is , there are no isolated molecules. Instead, each boron atom is surrounded by three oxygen atoms 120° apart, and these triangles are linked by sharing corner oxygens, as shown at the upper left.

The structure is not a regular crystal lattice, but is disordered. An orderly, solid array is a crystal, and a disordered array is known as a glass.

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