In copper oxide each of the copper atoms is expected to bond with two oxygen atoms. According to some theories however, the copper atoms can covalently bond with each other. Such bonds - in which the nuclei share electrons - are not usually associated with metals. Such behaviour could help explain high-temperature superconductivity in the cuprates, which all contain two-dimensional copper oxide planes. Covalent bonds increase the ability of a material to conduct electricity, unlike ionic bonds, which are poor conductors.