Which of the following statements correctly describes the difference between sigma (σ) and pi (π) molecular orbitals?

• A. Sigma (σ) molecular orbitals can only be formed from p orbitals.
• B. Pi (π) molecular orbitals have electron density along the internuclear axis.
• C. Sigma (σ) molecular orbitals result from head-on overlap of atomic orbitals, while pi (π) molecular orbitals result from side-by-side overlap.
• D. Pi (π) molecular orbitals are always lower in energy than sigma (σ) molecular orbitals.

Answer: (C)

Verified step by step guidance

1. Understand the nature of sigma (\sigma) and pi (\pi) molecular orbitals in terms of how atomic orbitals overlap to form them.
2. Recall that sigma (\sigma) molecular orbitals are formed by the head-on (axial) overlap of atomic orbitals, which can include s orbitals, p orbitals, or hybrid orbitals, resulting in electron density concentrated along the internuclear axis.
3. Recognize that pi (\pi) molecular orbitals are formed by the side-by-side (lateral) overlap of p orbitals only, with electron density located above and below the internuclear axis, not along it.
4. Note that the statement claiming sigma (\sigma) orbitals can only be formed from p orbitals is incorrect because s orbitals can also contribute to sigma bonding.
5. Understand that the relative energy of sigma and pi orbitals depends on the specific molecule and orbitals involved, so the statement that pi (\pi) orbitals are always lower in energy than sigma (\sigma) orbitals is not universally true.