Intermolecular Forces in Chemistry: Definition, Types & Examples

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Lesson Transcript
Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

This lesson defines the major forces that occur between molecules. Specifically, the lesson explains ion-dipole, dipole-dipole, and London (or dispersion) forces. Several examples are included to provide context.

What Are Intermolecular Forces?

Have you ever gently placed a paper clip or other small object on the surface of water? Did the paper clip float? If so, you have witnessed the result of the interactions of billions of water molecules that keep the surface of the liquid water from allowing the paper clip to sink! The interaction between molecules (substances that are made of atoms that bond through sharing of electrons to form covalent bonds) and other molecules or ions (atoms that have lost or gained electrons) are governed by physical forces. These forces, known as intermolecular forces, arise from the way electrons are shared within the covalent bonds of different molecules.

Two basic possibilities arise when considering the sharing of electrons between atoms that are covalently bonded together. Some molecules, like those found in petroleum or even olive oil, have equal sharing of electrons in their covalent bonds, known as non-polar molecules. Other molecules have unequal sharing of electrons (also known as polar molecules) between atoms where one atom pulls the electrons in the covalent bond closer to itself to form a partial negative charge, and the atoms in the molecule that have their electrons pulled away form a partial positive charge. The partial positive and negative charges in the molecule create a dipole.

Types of Intermolecular Interactions

Ion-dipole interactions occur when ions interact with polar molecules and represent the strongest of the forces that are discussed here. The charged ions are attracted to the dipole with the opposite partial charge. You have witnessed the results of this type of interaction if you have ever seen table salt dissolved into a glass of water. Table salt is made of positively charged sodium ions (Na^+) and negatively charged chloride ions (Cl^-). The sodium ions are attracted to the partial negative charge of the oxygen atoms in neighboring water molecules. The chloride ions are attracted to the partial positive charge on the hydrogen atoms of neighboring water molecules. Therefore, the attractive interactions allow the ions to be surrounded by water molecules and dissolve.

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