Intermolecular forces are attractive or repulsive forces that act between neighboring molecules or atoms. They are distinct from intramolecular forces, which are the chemical bonds that hold atoms within a molecule together. These forces are generally weaker than chemical bonds but play a crucial role in determining physical properties like melting points, boiling points, viscosity, and solubility.
London Dispersion Forces are present in all molecules, whether polar or nonpolar. They arise from temporary fluctuations in electron distribution around atoms or molecules, creating instantaneous dipoles. These temporary dipoles then induce dipoles in neighboring molecules, resulting in weak attractive forces. The strength of London Dispersion Forces increases with molecular size and the number of electrons in the molecule.
Dipole-dipole forces occur between polar molecules. These molecules have permanent dipoles due to uneven sharing of electrons between atoms with different electronegativities. The positive end of one polar molecule is attracted to the negative end of another polar molecule. These forces are stronger than London Dispersion Forces but weaker than hydrogen bonding. Examples include hydrogen chloride, carbon monoxide, and acetone.