In which of the following substances is the weakest intermolecular force expected? H2O, CH3, or OH? Explain this briefly or in detail.
The Correct Answer and Explanation is:
Correct Answer
The weakest intermolecular force is expected in CH₃.
Explanation
Intermolecular forces (IMFs) are the attractive or repulsive forces that exist between neighboring chemical species. The strength of these forces dictates many of a substance’s physical properties, such as boiling point and viscosity. The three main types of intermolecular forces, from strongest to weakest, are hydrogen bonding, dipole-dipole interactions, and London dispersion forces. To determine which substance has the weakest forces, one must analyze the molecular structure and polarity of each species.
H₂O (Water): The water molecule is composed of a highly electronegative oxygen atom bonded to two hydrogen atoms. This creates highly polar covalent bonds and results in a bent molecular geometry with a significant net dipole moment. Most importantly, the presence of hydrogen atoms directly bonded to an oxygen atom allows water molecules to form hydrogen bonds with each other. Hydrogen bonding is the strongest type of intermolecular force, leading to water’s unusually high boiling point and surface tension.
OH (Hydroxyl Radical): Similar to water, the hydroxyl radical contains a hydrogen atom bonded to a highly electronegative oxygen atom. This O-H bond is polar, and the species is capable of forming hydrogen bonds with other OH radicals. While its interactions might be less extensive than the three-dimensional network in water, the presence of hydrogen bonding still signifies a strong intermolecular attraction.
CH₃ (Methyl Radical): The methyl radical consists of a central carbon atom bonded to three hydrogen atoms. The electronegativity difference between carbon (2.55) and hydrogen (2.20) is very small. Consequently, the C-H bonds are considered nonpolar. With no significant separation of charge and a symmetrical trigonal planar geometry, the molecule itself is nonpolar. Nonpolar species do not exhibit dipole-dipole interactions or hydrogen bonding. Their only mode of intermolecular attraction is through London dispersion forces. These forces are temporary and arise from the random motion of electrons creating instantaneous, transient dipoles. As London dispersion forces are the weakest class of intermolecular forces, the attraction between CH₃ species is expected to be significantly weaker than the hydrogen bonding present in both H₂O and OH.
