A $\mathrm{p}i$-bond (pi bond) is a type of covalent bond formed by the sideways or lateral overlap of atomic orbitals. This type of overlap occurs when the orbitals are parallel to each other and perpendicular to the internuclear axis.

Why (B) is correct:

Pi bonds are typically formed by the sideways overlap of two p-orbitals. Each p-orbital has two lobes, and when two p-orbitals overlap sideways, the electron density is concentrated above and below the internuclear axis. This forms a pi bond.

Option Analysis:

• A) $s-s$ orbitals: Overlap of two s-orbitals always results in a sigma ($\sigma$) bond, which is formed by head-on or axial overlap.
• B) $p-p$ orbitals: Sideways overlap of two p-orbitals forms a pi ($\pi$) bond. This is the correct description.
• C) $s-p$ orbitals: Head-on overlap between an s-orbital and a p-orbital forms a sigma ($\sigma$) bond. Sideways overlap between an s-orbital and a p-orbital is not possible due to their different symmetries.
• D) $s-p-s$ orbitals: This combination is not a standard way to describe the formation of a single bond. Bonds are typically formed by the overlap of two atomic orbitals at a time.

Correct Answer: (B)