A(n) __ bond exhibits an unequal sharing of electrons between two or more molecules.
A. hydrogen
B. nonpolar covalent
C. ionic
D. polar covalent
The correct answer and explanation is:
Correct Answer: D. polar covalent
A polar covalent bond exhibits an unequal sharing of electrons between two or more atoms within a molecule. This type of bond typically forms between two nonmetal atoms that have different electronegativities. Electronegativity is a measure of how strongly an atom attracts electrons in a bond. When the difference in electronegativity between the two atoms is moderate (typically between 0.5 and 1.7), the electrons are not shared equally. Instead, they tend to be pulled more toward the atom with the higher electronegativity. This unequal distribution creates a dipole, meaning one end of the bond has a partial negative charge, and the other end has a partial positive charge.
For example, in a water molecule (H₂O), the oxygen atom is more electronegative than the hydrogen atoms. As a result, the shared electrons in each O-H bond are pulled more toward the oxygen atom, creating a partial negative charge near the oxygen and partial positive charges near the hydrogen atoms. This polarity leads to many of water’s unique properties, such as its high boiling point, surface tension, and ability to dissolve many substances.
The other choices do not involve this kind of unequal sharing. A hydrogen bond is a weak attraction between a hydrogen atom and an electronegative atom, not a sharing of electrons. A nonpolar covalent bond involves equal sharing of electrons between atoms with similar electronegativities, like in O₂ or H₂. An ionic bond, on the other hand, occurs when one atom completely transfers electrons to another, resulting in oppositely charged ions, as in NaCl.
In summary, polar covalent bonds are essential in biology and chemistry because they influence molecular interactions, solubility, and reactivity due to their partial charges and uneven electron distribution.