分子間力

Intermolecular forces, which exist between molecules, originate from electrostatic interactions between charges, partial charges, and temporary charges. All molecules produce temporary charges. Due to the varying distribution of electrons, a higher electron density in one region of the electron cloud results in an instantaneous dipole, or a temporary dipole. This subsequently induces another instantaneous dipole in the neighboring molecule. The domino effect of dipoles gives rise to weak intermolecular attractive forces called dispersion forces, which exist between all molecules, whether polar or nonpolar. Some covalent compounds, like water, for example, exhibit electron-rich and electron-poor regions caused by the atoms’ differences in electronegativity. The uneven distribution of shared electrons and the molecular shape of the compound create permanent partial charges, resulting in a permanent dipole in the otherwise neutral compound — making it polar. Molecules with permanent dipoles — also called polar compounds — align themselves through dipole–dipole forces, where the positive end of one molecule interacts electrostatically with the negative end of the neighboring molecule. If a polar compound contains a hydrogen atom covalently bonded to a small, highly electronegative atom like fluorine, oxygen, or nitrogen, the atoms tend to exhibit larger partial charges. Consequently, the hydrogen atom in F–H, O–H, or N–H bonds strongly interacts with the electronegative atom on its neighbor through a special type of dipole–dipole force called a hydrogen bond. Notably, hydrogen bonds are stronger than dipole–dipole forces, and compounds capable of forming hydrogen bonds exhibit higher melting and boiling points. The three intermolecular forces — dispersion, dipole–dipole, and hydrogen bonds — are, in comparison with intramolecular forces, relatively weak, with varying strengths. They collectively are classified as van der Waals forces. While dispersion forces are present between all molecules, polar or nonpolar, dipole–dipole forces and hydrogen bonds exist only around polar molecules. Exclusive to solutions is the ion–dipole force, which is the strongest intermolecular force. When an ionic compound like sodium chloride is dissolved in a polar solvent like water, the dissociated ions interact with the solvent's dipoles via strong ion–dipole forces. Here, the cations associate with the negative ends of the water molecules, while the anions interact with the positive ends.