The ability of an atom in a molecular entity to expand its @V06588@ shell beyond the limits of the @LT07065@. Hypervalent compounds are common for the second and subsequent row elements in groups 15–18 of the periodic table. A description of the hypervalent bonding implies a transfer of the electrons from the central (hypervalent) atom to the nonbonding molecular orbitals which it forms with (usually more electronegative) @L03518@. A typical example of the hypervalent bond is a linear three-centre, four-electron bond, e.g. that of Fap–P–Fap fragment of PF5.