ChemTalk

Ion-Dipole Forces

Core Concepts – Ion-Dipole Forces

In this tutorial, you will learn about the ion-dipole forces as well as the ion-induced dipole forces.

Topics Covered in Other Articles

Vocabulary

Ion – an atom or a molecule that bears a positive or negative charge(s) due to the loss or gain of one or more electrons.

Dipole – a molecule with two poles with opposite charges, separated by a distance.

Electrostatic interaction – an attractive or repulsive interaction between electrically charged molecules.

Intermolecular force – a physical force between molecules.

Ion-Dipole Forces

An ion-dipole force is an electrostatic interaction between a fully charged ion and a neutral molecule that has a dipole. Moreover, it is an intermolecular force that is most commonly found in solutions, especially ionic compounds dissolved in polar liquids.

The charge of the ion determines which part of the molecule attracts and repels. Therefore, a cation attracts the partially negative end of a molecule, whereas an anion attracts the partially positive end of a molecule.

Example of Ion-Dipole Forces:

Na+ (sodium ion) and H2O (water molecule)

Na+: a cation

Partially positive end of H2O: from H atoms

Partially negative end of H2O: from O atom

→ Sodium attracts the oxygen atom of the water molecule, while it repels the hydrogen atom.

Ion-dipole forces in sodium
wikipedia.org

Ion-Induced Dipole Forces

An ion-induced dipole force is a weak attraction that results when an ion interacts with a nonpolar molecule. Moreover, when a fully charged ion approaches a nonpolar molecule, the electrons respond and the molecule forms a dipole; therefore, the ion polarizes the nonpolar molecule.

A cation polarizes the molecule by attracting the electron cloud, while an anion does it by repelling the electron cloud.

Example of Ion-Induced Dipole Forces:

NO3– (nitrate ion) and I2 (iodine molecule)

NO3–: an anion

I2: nonpolar molecule

→ In the presence of a nitrate ion, iodine molecule gets polarized.