A molecule with a polar covalent bond between atoms exhibits

Analysis

The question asks for the defining characteristic of a polar covalent bond. To answer this, we must understand how electrons behave when two atoms with different electronegativities form a bond.

Concept Explanation

• Covalent Bond: This is a chemical bond formed by the sharing of electron pairs between atoms.
• Non-polar Covalent Bond: This occurs when the atoms involved have similar electronegativities (e.g., $H_2$ or $Cl_2$). The electrons are shared equally because neither atom has a stronger "pull" on the electrons.
• Polar Covalent Bond: This occurs when there is a significant difference in the electronegativity of the two atoms (e.g., $HCl$ or $H_2O$). The more electronegative atom attracts the shared electron pair more strongly toward itself. Because the electrons spend more time near the more electronegative atom, the sharing is unequal, resulting in a partial negative charge ($\delta-$) on one atom and a partial positive charge ($\delta+$) on the other.

Step-by-Step Solution

1. Identify the nature of the bond: A polar covalent bond is characterized by a dipole moment created by the uneven distribution of electron density.
2. Evaluate the options: • Option A: "Unequal sharing of electron pair" accurately describes the shift of electron density toward the more electronegative atom. • Option B: "Equal sharing" describes a non-polar covalent bond. • Options C & D: These refer to "lone pairs," which are non-bonding electrons. While lone pairs affect molecular geometry, the definition of a polar covalent bond specifically concerns the shared bonding pair.

Conclusion

The unequal sharing of the bonding electron pair is the fundamental reason why a covalent bond becomes polar.

Exam Tip: Remember the mnemonic "P-U-E": Polar = Unequal sharing of Electrons. If you see a bond between two different non-metals (like $C$ and $O$), it is almost always polar because they have different electronegativity values!