Concept: Molecular Orbital Theory and Overlap of Atomic Orbitals

Molecular orbitals are formed by the linear combination of atomic orbitals (LCAO). The type of molecular orbital formed depends on the symmetry of the overlapping atomic orbitals and the internuclear axis. For a diatomic molecule, the internuclear axis is conventionally taken as the z-axis.

Why (A) is correct:

When atomic p-orbitals overlap sideways, they form $\pi$-molecular orbitals. The $p_x$ and $p_y$ orbitals are perpendicular to the internuclear (Z) axis. Therefore, when two $p_x$ orbitals (or two $p_y$ orbitals) from different atoms approach each other along the Z-axis, they overlap sideways. This sideways overlap leads to the formation of $\pi$-molecular orbitals (both bonding and antibonding).

Option Analysis:

• A) $\pi$-molecular orbital: This is correct. Sideways overlap of $p_x$ or $p_y$ orbitals along the Z-axis forms $\pi$-molecular orbitals.
• B) $\sigma$-molecular orbital: $\sigma$-molecular orbitals are formed by head-on (axial) overlap of atomic orbitals (e.g., s-s, s-p_z, p_z-p_z). Since the main axis is Z, $p_z$ orbitals would form $\sigma$ bonds, not $p_x$ or $p_y$.
• C) $\delta$-molecular orbital: $\delta$-molecular orbitals are formed by the overlap of four lobes of d-orbitals (e.g., $d_{xy}$ or $d_{x^2-y^2}$ orbitals). This is not relevant for p-orbital overlap.
• D) No bond will form: This is incorrect. $p_x$ and $p_y$ orbitals do overlap to form $\pi$-bonds.

Correct Answer: (A)