Intermolecular forces (IMFs) are the weak attractive forces between individual molecules that fundamentally determine a substance’s bulk physical properties. These forces are distinct from the much stronger intramolecular forces, which are the chemical bonds holding atoms together within a molecule. The strength of these attractions dictates characteristics like boiling point, melting point, and viscosity, as energy must be supplied to overcome them during phase changes.
The Basis of Intermolecular Forces
The presence and strength of intermolecular forces rely on a molecule’s electrical landscape, shaped by its bond polarity and overall geometry. Bond polarity arises from the difference in electronegativity between two bonded atoms, creating a localized charge separation called a bond dipole. Molecular geometry determines if these individual bond dipoles cancel out or combine to form a net molecular dipole moment. If a molecule has a symmetrical shape (e.g., tetrahedral with identical outer atoms), the dipoles cancel, resulting in a net dipole moment of zero and classifying the molecule as nonpolar. Conversely, an asymmetrical arrangement leads to a permanent molecular dipole moment and a polar molecule. The three main types of intermolecular forces are London Dispersion Forces (LDF), Dipole-Dipole forces, and Hydrogen Bonding.
Intermolecular Forces in Carbon Tetrafluoride (\(\text{CF}_4\))
Carbon tetrafluoride (\(\text{CF}_4\)) has a central carbon atom bonded to four fluorine atoms. The significant electronegativity difference means each of the four \(\text{C-F}\) bonds is highly polar, possessing a strong bond dipole pointing toward fluorine. \(\text{CF}_4\) adopts a perfectly symmetrical tetrahedral geometry. Because the four identical \(\text{C-F}\) bonds are arranged symmetrically, their individual bond dipoles cancel each other completely. This results in a net molecular dipole moment of zero, classifying \(\text{CF}_4\) as a nonpolar molecule.
Since \(\text{CF}_4\) is nonpolar, the only intermolecular force acting between its molecules is the London Dispersion Force (LDF). LDFs arise from the constant, random motion of electrons, which momentarily creates a temporary, instantaneous dipole that can induce a corresponding dipole in an adjacent molecule.
Intermolecular Forces in Fluoromethane (\(\text{CH}_3\text{F}\))
Fluoromethane (\(\text{CH}_3\text{F}\)) has a central carbon atom bonded to three hydrogen atoms and one fluorine atom. This composition fundamentally alters its electrical behavior compared to \(\text{CF}_4\). The \(\text{C-F}\) bond is highly polar due to the large electronegativity difference. The three \(\text{C-H}\) bonds are only slightly polar.
Because the four bonds around the central carbon atom are not identical, the molecular symmetry is destroyed. The highly polar \(\text{C-F}\) bond dipole is not balanced by equal and opposite dipoles, resulting in a permanent net molecular dipole moment. This makes \(\text{CH}_3\text{F}\) a polar molecule. Consequently, the pure substance exhibits two types of intermolecular forces: London Dispersion Forces and Dipole-Dipole forces. Dipole-Dipole forces are the electrostatic attractions between the partial positive end of one molecule and the partial negative end of a neighboring molecule.
Comparing the Strength and Effects of the Forces
The difference in molecular polarity leads to a significant difference in the intermolecular forces and resulting physical properties of the two compounds. \(\text{CF}_4\) relies solely on the relatively weak London Dispersion Forces (LDF). \(\text{CH}_3\text{F}\), being a polar molecule, experiences LDF plus the stronger Dipole-Dipole forces.
The presence of this additional interaction means the collective attractive forces holding \(\text{CH}_3\text{F}\) molecules together are substantially greater than the forces in \(\text{CF}_4\). Overcoming these stronger attractions requires a greater input of energy, which is demonstrated by their normal boiling points. The boiling point of \(\text{CF}_4\) is approximately \(-128^\circ\text{C}\). In contrast, the boiling point of \(\text{CH}_3\text{F}\) is significantly higher, around \(-78^\circ\text{C}\), reflecting the energy needed to overcome both LDF and the stronger Dipole-Dipole attractions.