What Are Intermolecular Forces?

C.H. Seman

In chemistry, intermolecular forces describe various electrostatic forces present between atoms and molecules. These forces include ion-dipole forces, hydrogen bonding, dipole-dipole forces and London dispersion forces. Although these forces are generally much weaker than ionic or covalent bonds, they may still have a large influence on the physical characteristics of liquids, solids or solutions.

Intermolecular forces describe various electrostatic forces present between atoms and molecules.
Intermolecular forces describe various electrostatic forces present between atoms and molecules.

All intermolecular forces are electrostatic in nature. This means the mechanics of these forces depend on the interactions of charged species such as ions and electrons. Factors relating to electrostatic forces, such as electronegativity, dipole moments, ion charges and electron pairs, can greatly affect the types of intermolecular forces between any two given chemical species.

Ion-dipole forces are present between ions and the partial charges at the ends of polar molecules. Polar molecules are dipoles and have a positive end and a negative end. Positively charged ions are attracted to the negative end of a dipole and negatively charged ions are attracted to the positive end of a dipole. The strength of this kind of intermolecular attraction increases with increasing ion charge and increasing dipole moments. This particular kind of force is commonly found in ionic substances dissolved in polar solvents.

For neutral molecules and atoms, the intermolecular forces that may be present include dipole-dipole forces, hydrogen bonding and London dispersion forces. These forces make up the van der Waals forces, which are named after Johannes van der Waals. In general, they are weaker than ion-dipole forces.

Dipole-dipole forces occur when the positive end of a polar molecule approaches the negative end of another polar molecule. The force itself depends on the proximity of the molecules. The farther away the molecules are, the weaker the dipole-dipole forces. The force's magnitude also may increase with increasing polarity.

London dispersion forces can occur between both nonpolar and polar chemical species. They are named in honor of their discoverer, Fritz London. The force itself occurs because of the formation of instantaneous dipoles; these can be explained by the movement of electrons in the chemical species.

Instantaneous dipoles are created when electrons around one chemical species are attracted to the nucleus of another chemical species. In general, London dispersion forces are greater for larger molecules, because larger molecules have more electrons. Large halogens and noble gases, for instance, have higher boiling points than small halogens and noble gases because of this.

Hydrogen bonds occur between hydrogen atoms in a polar bond and unshared electron pairs on small electronegative ions or atoms. This type of intermolecular force is often seen between hydrogen atoms and fluorine, oxygen or nitrogen. Hydrogen bonds can be found in water and are responsible for water's high boiling point.

Intermolecular forces can have a profound effect on the physical characteristics of a chemical species. Typically, high boiling points, melting points and viscosity are associated with high intermolecular forces. Although they are much weaker than covalent and ionic bonds, these forces of intermolecular attraction are still important in describing the behavior of chemical species.

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