Intermolecular vs Intramolecular Forces

Chemical substances form when atoms or ions interact electrostatically.

For example, one oxygen atom and two hydrogen atoms covalently bond to form a molecule of water. Such bonding forces that hold the atoms together within a molecule are called intramolecular forces.

Intramolecular forces dictate chemical properties like stability and types of chemical bonds. The three basic types are ionic, covalent, and metallic bonds.

An ionic bond is formed by the transfer of valence electrons from a metal to a nonmetal atom, which results in an electrostatic attraction between the oppositely charged ions.

A covalent bond is formed when nonmetal atoms share their valence electrons.

Lastly, metallic bonding results from the interaction between the array of positive metal ions and a shared pool of delocalized valence electrons.

However, electrostatic interactions do not only exist within a molecule but also between molecules.

For example, in water—whether solid, liquid, or gaseous—the molecules interact via electrostatic, nonbonding interactions dictating the state of matter. These interactions are called intermolecular forces and influence various physical properties, such as melting and boiling points.

Intermolecular forces can be categorized into several types. Strong ion–dipole forces occur between ions and polar molecules; dipole–dipole forces exist between polar molecules, with hydrogen bonding being a special type of dipole–dipole force; and finally, the weakest of all—dispersion forces—exist in all molecules, polar and nonpolar, and are a result of temporary dipoles.

Intermolecular forces are weak because small or partial charges are interacting over large distances, as compared to intramolecular forces, which are strong owing to large electrostatic interactions over short distances.

For example, in liquid water, the molecules are separated by an average distance of about 300 picometers, characteristic of the comparatively weaker intermolecular forces.

Consequently, it takes heating water to only 100 °C to overcome these intermolecular forces and transition liquid-phase water molecules into the vapor phase.

In contrast to this, the length of the O–H bond in water is 96 picometers, characteristic of the stronger intramolecular bonds. It takes heating water to around 1000 °C, much more than its boiling point, to break this intramolecular bond.