Answer
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The reason why atoms in a single bond can rotate about the internuclear axis without breaking the bond, while atoms in a double or triple bond cannot rotate about the internuclear axis unless the bond is broken, is due to the differences in the bond types and their associated electron density distributions.
In a single bond, the atoms are connected by a single covalent bond, which is formed by the sharing of a pair of electrons between the two atoms. This single bond is primarily composed of a sigma (σ) bond, which is a type of covalent bond that is formed by the head-on overlap of atomic orbitals. The sigma bond allows for the rotation of the atoms around the internuclear axis without breaking the bond, as the electron density is evenly distributed and can accommodate the rotation.
In contrast, double and triple bonds involve the formation of additional pi (π) bonds, which are formed by the side-to-side overlap of atomic orbitals. The pi bonds contribute to the overall bond strength and stability, but they also restrict the rotation of the atoms around the internuclear axis. This is because the pi bonds have a specific orientation and distribution of electron density, which would be disrupted if the atoms were to rotate.
If the atoms in a double or triple bond were to rotate about the internuclear axis, the pi bonds would be broken, and the overall bond strength would be significantly reduced. This would require the input of a significant amount of energy to overcome the stability of the double or triple bond, effectively breaking the bond altogether.
Therefore, the atoms in a single bond can rotate about the internuclear axis without breaking the bond, while the atoms in a double or triple bond cannot rotate about the internuclear axis unless the bond is broken, due to the differences in the bond types and their associated electron density distributions.
