Intermolecular vs Intramolecular Forces

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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.

Overview

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Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular force is influenced by the distance of separation between molecules. The forces significantly affect the interactions in solids and liquids, where the molecules are close together. In gases, IMFs become important only under high-pressure conditions (due to the proximity of gas molecules). Intermolecular forces dictate the physical properties of substances, such as their melting point, boiling point, density, and enthalpies of fusion and vaporization. When a liquid is heated, the thermal energy acquired by its molecules overcomes the IMFs that hold them in place, and the liquid boils (converts into the gaseous state). Boiling points and melting points depend on the type and strength of the intermolecular forces. For example, a high boiling liquid, like water (H₂O, b.p. 100 °C), exhibits stronger intermolecular forces compared to a low boiling liquid, like hexane (C₆H₁₄, b.p. 68.73 °C).

While intermolecular forces exist between molecules, intramolecular forces exist within molecules and hold the atoms in a given molecule together. Intramolecular forces keep a molecule intact; a change in the state of a substance does not affect intramolecular interactions. For example, although the melting of ice partially disrupts the intermolecular forces between solid H₂O molecules, thereby rearranging them and converting ice into liquid water, it does not break down individual H₂O molecules.

Intramolecular forces may be ionic, covalent, or metallic in nature.

Atoms gain (nonmetals) or lose electrons (metals) to form ions (anions and cations) with particularly stable electron configurations. Compounds composed of ions are called ionic compounds (or salts), and their constituent ions are held together by ionic bonds: electrostatic forces of attraction between oppositely charged cations and anions. For example, magnesium chloride (MgCl₂) is an ionic compound composed of magnesium cations and chloride anions held together by strong ionic bonds.

A covalent bond (nonpolar or polar) is formed when electrons are shared between atoms, and a molecule is formed. Nonpolar covalent bonds arise when atoms share electrons equally, such as in hydrogen (H₂). Polar covalent bonds form due to unequal sharing of electrons; one atom exerts a stronger force of attraction on the electrons than the other. An example is hydrogen chloride, HCl.

Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms. The atoms within such a metallic solid are held together by a unique force known as metallic bonding that gives rise to many useful and varied bulk properties.

Intermolecular forces are much weaker compared to intramolecular forces. For example, to overcome the IMFs in one mole of liquid HCl and convert it into gaseous HCl requires only about 17 kilojoules. However, to break the covalent bonds between the hydrogen and chlorine atoms in one mole of HCl requires about 25 times more energy, which is 430 kilojoules.

This text is adapted from Openstax, Chemistry 2e, Chapter 10: Liquids and Solids.

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