Difference Between Dipole Dipole and London Dispersion Forces

Main Difference – Dipole Dipole vs London Dispersion Forces

There are two types of forces between molecules and atoms: primary bonds and secondary bonds. Primary bonds are the chemical bonds that occur between atoms and can be categorized as ionic, covalent and metallic bonds. These bonds are also called intramolecular bonds. Secondary forces are attractive forces occurring between molecules. Thus, they are called intermolecular forces. There are three main types of secondary bonds: dipole-dipole, London dispersion and hydrogen bonds. Hydrogen bond is a special type of dipole-dipole attraction that occurs between a lone pair of electrons on an electronegative atom and a hydrogen atom in a polar bond. The main difference between dipole-dipole and London dispersion forces is that dipole-dipole forces occur among molecules with dipole moment whereas London dispersions occur due to instantaneous dipoles that form in atoms or nonpolar molecules.

This article explains, 

1. What are Dipole Dipole Forces?
     – Definition, Features, Characteristics, Examples

2. What are London Dispersion Forces?
     – Definition, Features, Characteristics, Examples

3. What is the difference between Dipole Dipole and London Dispersion Forces?

What are Dipole-Dipole Forces

Dipole-dipole forces occur when there is an unequal sharing of electrons between two atoms. Unequal sharing of electrons results in opposite charges on the parent atom, forming permanent dipoles. These dipoles attract each other and form dipole-dipole forces. The molecules with dipole moments are known as polar molecules. The strength of a dipole moment of a molecule is proportional to the strength of a dipole-dipole force. Hydrogen bond is a special type of dipole-dipole force. Dipole-diploe forces can be seen among molecules like water, HCl, etc. These forces do not occur among molecules with a zero dipole movement.

Dipole-dipole interaction in HCl

What are London Dispersion Forces

London dispersion forces occur when a positively charged nucleus of an atom attracts the electron cloud of another atom. When electron clouds of both atoms are brought together due to the same charge, the electrons clouds mutually repel one another. Because of the nearness of electron clouds, temporary dipoles known as instantaneous dipoles are formed. These dipoles occur due to the unsymmetrical motion of electron around the nuclei of atoms. London dispersion forces can occur among both polar and non-polar molecules, among ions, and among the single atoms of noble gases. The influence of London dispersion forces are ignored in metals, ionically bonded compounds, and in large covalent solids. However, these forces are significantly considered in molecules with dipole-dipole forces. It is because the bond energies of dispersion forces are much higher than that of dipole-dipole forces.

Difference Between Dipole Dipole and London Dispersion Forces

Definition

Dipole-dipole forces: Dipole-dipole forces are attractive forces between molecules with permanent dipole movements.

London dispersion forces: London dispersion forces are attractive forces between all kinds of molecules including polar, non-polar, ions, and noble gasses.

Formation

Dipole-dipole forces: Dipole-dipole forces occur when there is an unequal sharing of electrons between two atoms.

London dispersion forces: London dispersion forces occur when a positively charged nucleus of an atom attracts the electron cloud of another atom.

Bond strength

Dipole-dipole forces: Dipole-dipole forces have a weaker bond strength.

London dispersion forces: London dispersion forces have a higher bond strength.

Dipole moment

Dipole-dipole forces: Permanent dipoles must exist.

London dispersion forces: Instantaneous dipoles must exist.

References: