The strength of polar-nonpolar interactions is affected by the atomic size of the nonpolar atom (or molecule) and the electronegativity of the polar molecule.
Significance of the Atomic Size in Nonpolar Atoms
An atom gets bigger due to the addition of more electron-containing shells; therefore, the size increase directly relates to more electron number. Such bigger atoms can undergo distortion of their electron cloud since they are less influenced by the attractive pull from the distant nucleus. The distortion makes it easier to form temporary poles in nonpolar atoms. These poles then interact with the innate poles of the polar molecules and cause stronger attractions, also known as the Debye Force.
For example, between two nonpolar atoms, Helium (He) and Argon (Ar), a polar molecule like HCl will interact strongly with bigger, more polarisable Ar than smaller He. The few electrons of He are strongly pulled by the nucleus and, therefore, less engaged in polarization.
Significance of Electronegativity in Polar Molecules
If the electronegativity difference (0.5-1.7) between two heteroatoms in a molecule is greater, it will induce polarity in the nonpolar atom more strongly, leading to more efficient polar-nonpolar interactions.
For example, between two polar molecules, HF and HCl, the interaction of HF with He will be stronger than the HCl-He pair. The F is more electronegative than Cl; therefore, HF will strongly influence and distort the electron cloud of He, inducing dipoles more easily than HCl.
The above excerpt supports the chapter Intermolecular Forces part of CurlyArrows' Introductory Organic Chemistry Course. Preview the Book.
Related Reading- Debye Force, What is electronegativity?
