Intermolecular and Intramolecular Forces in Chemistry: Properties of Solids and Liquids

Intermolecular Forces

These forces exist between molecules and influence the physical properties of the substance.

Ion-ion Interactions

The force of attraction between two oppositely charged ions governed by Coulomb's law.

Coulomb's Law

Determines the melting and boiling points of ionic compounds; higher attraction forces lead to higher melting and boiling points.

Ion-dipole Interactions

Attractive force between an ion and a polar molecule.

Dipole-dipole Interactions

Occurs only for molecular compounds that are polar.

Hydrogen Bonding

A special type of dipole-dipole interaction present in molecules having hydrogen atoms attached to N, O, F.

London Dispersion Forces

Also called van der Waals interaction; the weakest of the intermolecular forces present in all molecules.

Trend for IMFA Strength

Ion-ion (intramolecular) > ion-dipole > H-bonding > dipole-dipole > London dispersion force.

Molar Mass and Boiling Point

Generally, the stronger the intermolecular forces, the higher the boiling point.

Molecular Shape and Boiling Point

Branched versus linear structures affect boiling points.

Higher Charge and Melting Points

The higher the charge of the ion, the higher the melting/boiling points.

Smaller Ions and Melting Points

For ions with similar charge, smaller ions have higher melting/boiling points.

Polar Molecules

Molecules that have a dipole moment due to the presence of polar bonds.

Non-polar Molecules

Molecules that do not have a dipole moment and only exhibit London dispersion forces.

Weakest Intermolecular Force

London Dispersion Forces are the weakest of the intermolecular forces.

Strongest Intermolecular Force

Ion-ion interactions are the strongest intermolecular forces.

Electronegativity

The tendency of an atom to attract a bonding pair of electrons.

Physical Properties Influenced by IMFA

Boiling point, melting point, and solubility are influenced by the strength of intermolecular forces.

IMFA strength

Leads to higher boiling point, lower vapor pressure, stronger surface tension, and higher viscosity.

Crystalline solids

Have well-defined structures due to orderly arrangement of particles.

Amorphous solids

Do not have a well-ordered molecular structure; examples include waxes, glasses, and asphalt.

Molecular solids

Have molecules in the lattice positions, held by IMFAs, with low melting points and are volatile.

Ionic solids

Have ions in the unit cell, with high melting points, low conductivity, and are hard and brittle.

Network covalent solids

Linked by strong covalent bonds, with extremely high melting and boiling points.

Metallic solids

Held together by metallic bonding, with high electrical and thermal conductivity.