Which of the following best describes a polar covalent bond?
A. Electrons are shared unequally between two atoms.
B. Electrons are shared equally between two atoms.
C. Electrons are transferred from one atom to another to generate oppositely charged ions that are attracted to one another.
D. Brief fluctuations in electron density create oppositely charged regions in two atoms that then attract one another
The Correct answer and Explanation is:
The correct answer is A. Electrons are shared unequally between two atoms.
Explanation:
A polar covalent bond occurs when two atoms share electrons, but the sharing is not equal. This unequal sharing arises due to the difference in electronegativity between the two atoms involved in the bond. Electronegativity is a measure of an atom’s ability to attract and hold onto electrons. When two atoms with different electronegativities form a bond, the atom with the higher electronegativity attracts the shared electrons more strongly than the atom with lower electronegativity.
For instance, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms. As a result, the shared electrons spend more time closer to the oxygen atom than to the hydrogen atoms. This creates a partial negative charge (δ-) at the oxygen end of the molecule and a partial positive charge (δ+) at the hydrogen ends. This charge separation leads to the formation of a dipole moment, where one end of the molecule is slightly negative and the other end is slightly positive.
This characteristic of polar covalent bonds significantly influences the physical properties of substances. For example, water’s polarity allows it to dissolve many ionic and polar substances, making it an excellent solvent. Moreover, the polarity of water leads to its high boiling and melting points compared to similar-sized nonpolar molecules, due to the hydrogen bonding that occurs between water molecules.
In contrast, answer choices B and C describe nonpolar covalent bonds and ionic bonds, respectively. Answer D refers to temporary dipoles, which are characteristics of London dispersion forces and not covalent bonds. Thus, option A is the most accurate description of a polar covalent bond, emphasizing the unequal sharing of electrons between atoms.