Molecular Polarity and Intermolecular Forces

Molecular polarity and intermolecular forces determine how molecules interact with each other and with biological environments. These concepts are key to understanding solubility, phase behavior, and membrane transport.

Molecular Geometry Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, regions of electron density repel one another and arrange themselves to minimize repulsion. These arrangements help determine molecular shape. Molecular geometry affects:

Common Molecular Shapes

Molecular Polarity A molecule is polar if it has an uneven distribution of electron density, creating a net dipole moment. Polarity depends on both bond polarity and molecular geometry. Symmetrical molecules with polar bonds may be nonpolar overall (e.g., CO₂).

CO₂: C=O bonds are polar → molecule is linear and symmetrical → bond dipoles cancel → nonpolar molecule

H₂O: O–H bonds are polar → molecule is bent → dipoles do not cancel → polar molecule

Polar substances generally dissolve better in polar solvents, while nonpolar substances generally dissolve better in nonpolar solvents. This is summarized by the idea of "like dissolves like."

Intermolecular Forces Intermolecular forces (IMFs) are attractive forces between molecules. Although weaker than covalent or ionic bonds, they strongly influence a substance's physical properties including:

London Dispersion Forces London dispersion forces result from temporary fluctuations in electron distribution that create temporary dipoles.

Strength increases with:

Dipole-Dipole Forces Dipole-dipole forces occur between polar molecules. The partially positive (δ⁺) end of one molecule is attracted to the partially negative (δ⁻) end of another.

Hydrogen Bonding Hydrogen bonding is a strong special type of dipole-dipole attraction. It occurs when hydrogen is directly bonded to nitrogen (N), oxygen (O), or fluorine (F). The partially positive hydrogen is attracted to the lone pair on N, O, or F of a nearby molecule.

Requirements:

Examples:

Ion-Dipole Forces Ion-dipole forces occur between a charged ion and a polar molecule. Examples include Na⁺ + H₂O and Cl⁻ + H₂O. Water molecules surround dissolved ions, forming hydration shells and allowing many ionic compounds to dissolve in water.

Van der Waals Forces Van der Waals forces is a broad term used to describe weak intermolecular attractions. Van der Waals forces generally include:

Physical Properties and Intermolecular Forces (IMFs)

Boiling point: temperature at which a liquid's vapor pressure equals the external pressure.

Stronger intermolecular attractions generally require more energy to separate molecules, leading to higher boiling points.

Vapor pressure: pressure exerted by a vapor in equilibrium with its liquid (or solid) in a closed system.

Volatility: how readily a substance vaporizes.

Stronger IMFs generally →

Water as a Solvent Water is a polar molecule that forms hydrogen bonds. It dissolves ionic and polar substances well but not nonpolar substances. This property is summarized by the rule "like dissolves like."

"Like dissolves like": polar substances generally dissolve best in polar solvents, while nonpolar substances generally dissolve best in nonpolar solvents.