91Ó°ÊÓ

A bond between two atoms is formed when their valence (outer) electrons overlap. A single bond consists of one pair of shared electrons and is called a sigma bond. A double bond consists of two pairs of shared electrons: one sigma bond and one pi bond. A triple bond consists of three pairs of shared electrons: one sigma bond and two pi bonds. Sigma bonds are formed from the direct, end-to-end overlap of orbitals. This allows the atoms in the single bond to rotate about the internuclear axis, as the orbitals maintain their overlapping configuration. However, pi bonds are formed from the parallel, side-to-side interaction of orbitals. Since the overlapping regions in a pi bond are above and below the internuclear axis, rotating the atoms about this axis would disrupt the overlapping regions, breaking the bond.

Sigma bonds are generally stronger than pi bonds due to the higher degree of overlap of the orbitals involved. This allows for rotation about the internuclear axis without breaking the bond, as the atoms remain connected and the electrons remain shared. On the other hand, pi bonds have a more limited overlap of orbitals, which are parallel to the internuclear axis. This makes them more susceptible to disruption upon rotation, which would break the bond.

In a single bond, only a sigma bond is present. Due to its stronger and more direct orbital overlap, rotation about the internuclear axis is possible without breaking this bond. However, in a double or triple bond, the presence of pi bonds formed by parallel, side-to-side overlap of orbitals makes rotation about the internuclear axis impossible without breaking the bond. Since these pi bonds are weaker than sigma bonds and have a more limited orbital overlap, rotating the atoms would disrupt the bond's electron sharing, leading to breaking of the bond. In summary, single bonds allow rotation about the internuclear axis due to their strong, direct orbital overlap in sigma bonds, while multiple bonds do not allow rotation because of the weaker, more vulnerable pi bonds formed by side-to-side orbital overlap.