Chemical Bonding and Structure - Intermolecular forces

Intermolecular forces (IMFs) are the forces of attraction between molecules, which determine physical properties such as boiling point, melting point, and solubility. They are much weaker than intramolecular forces like covalent or ionic bonds.

London (Dispersion) Forces: The weakest IMF, present in all molecules. They arise from temporary dipoles caused by the movement of electrons. The strength of LDFs increases with increasing molar mass ($M_r$) and larger surface area (e.g., $I_2$ has stronger LDFs than $F_2$).

Dipole-Dipole Forces: Forces of attraction between the permanent dipoles of polar molecules. A molecule is polar if it has polar bonds (difference in electronegativity $\Delta \chi > 0.4$) and a non-symmetrical geometry (e.g., $HCl$, $SO_2$).

Hydrogen Bonding: A specific, strong type of dipole-dipole attraction. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom ($N$, $O$, or $F$) and is attracted to the lone pair of an $N$, $O$, or $F$ atom in a neighboring molecule.

The relative strength of IMFs generally follows the order: $\text{London Dispersion} < \text{Dipole-Dipole} < \text{Hydrogen Bonding}$. However, very large molecules with only LDFs can sometimes have higher boiling points than small molecules with hydrogen bonding.

Solubility: 'Like dissolves like.' Polar solutes (like $C_6H_{12}O_6$) dissolve in polar solvents (like $H_2O$) due to favorable dipole-dipole or hydrogen bonding interactions. Non-polar solutes (like $I_2$) dissolve in non-polar solvents (like $C_6H_{14}$) via LDFs.