A Level Chemistry OCR Practice Exam

The defining characteristic of a polar covalent bond is that it exhibits a permanent dipole. This occurs when there is a significant difference in electronegativity between the two atoms involved in the bond. In such cases, one atom attracts the shared electrons more strongly than the other, leading to an uneven distribution of electron density. As a result, one end of the bond becomes slightly negative (the atom with the higher electronegativity), while the other end becomes slightly positive (the atom with the lower electronegativity). This creates a dipole moment, which is a measure of the separation of positive and negative charges within the bond.

The other options do not accurately define polar covalent bonds: the movement of electrons pertains more to conductive materials, a purely ionic bond involves complete transfer of electrons, and bonds between two metals are typically metallic bonds rather than covalent in nature. Thus, the presence of a permanent dipole distinctly characterizes polar covalent bonds.