Molecular orbital
In the vapour phase, beryllium oxide is present as discrete diatomic molecules. In the language of valence bond theory, these molecules can be described as adopting "sp" orbital hybridisation on both atoms, featuring one σ (between one "sp" orbital on each atom) and one π bond (between aligned "p" orbitals on each atom oriented perpendicular to the molecular axis). Molecular orbital theory provides a slightly different picture with no "net" sigma bonding (because the 2"s" orbitals of the two atoms combine to form a filled sigma bonding orbital and a filled sigma* anti-bonding orbital) and two pi bonds formed between both pairs of "p" orbitals oriented perpendicular to the molecular axis. The sigma orbital formed by the "p" orbitals aligned along the molecular axis is unfilled. The corresponding ground state is ...(2sσ)2(2sσ*)2(2pπ)4 (as in the isoelectronic C2 molecule), where both bonds can be considered as dative bonds from oxygen towards beryllium.