Dispersion Forces

Introduction

Dispersion force arises because of the fluctuation in the polarizability. It is a temporary effect. The noble gases that don't form any chemical bonds are attracted to each other and condense to liquid form at low temperatures. So, atoms also have a type of attraction. Fritz London, an Austrian physicist in 1930 introduced a theory that states that when two molecules approach each other the flow of electrons or the movement of electrons in one molecule will interfere with the electron movement in the other molecule. And they are by resulting in an attractive force.

For a better understanding of this theory, a Helium molecule is taken and is then approached with a dipole. It is observed that the electrons present in the Helium atom get attracted to the positive end of the dipole. So, it indicates that the Helium atom is polarized because of the presence of the dipole molecule so an induced dipole is developed in the Helium atom.

London Forces Definition

For all the atoms and molecules dispersion forces are present. The weak intermolecular force of attraction between two atoms or molecules when they approach close enough is the London force. And this force is generated because of the approach of electrons present in the two molecules. So, it is a quantum force. It is a weak van der Waals force that will result in the condensation of non-polar molecules into liquids. The effects may also be present in atoms, but it is more predominant in the case of compounds as it contains a different type of atoms. Because of this effect, a temporary polarization result. And this force is well observed in easily polarizable molecules.

The effect of this force will depend on the size of the molecule. For small molecules, this force is very weak while for large molecules this force is very strong. And the reason for this effect is because for larger molecules the electrons present on them are a larger distance from the nucleus compared to small molecules where the electrons are closely associated with the nucleus. Since a molecule is getting polarized, it will affect the melting and boiling points too.

• For example, considering halogen molecules chlorine, $\mathrm{Cl_{2}}$ and bromine, $\mathrm{Br}$. Even though both are halogen molecules their properties are different and are because of the London force. Chlorine is a gas while bromine is a liquid. As we know bromine is a larger molecule than chlorine the effective London force will be much higher in bromine, and it makes it condense to form liquids.

Hydrogen bonding

Hydrogen bonding is also a type of intermolecular interaction. This force is stronger than the van der Waals force. The hydrogen atom bonded with an electronegative atom has a tendency or a high affinity toward electrons present in an atom that has a lone pair of electrons. And it will further result in the formation of a bond and this bond is called the hydrogen bond. And is because of the reason that hydrogen atom with an electronegative atom has a positive charge on it. Atoms always tend to lose their charges by taking electrons. Water, $\mathrm{H_{2}O}$ is the best example of a molecule that is showing hydrogen bonding because it contains a hydrogen atom attached to the electronegative atom, oxygen. And is the reason why water molecules are present in liquid form in a large range of temperatures.

OpenStax College, 210 Hydrogen Bonds Between Water Molecules, CC BY 3.0

The double helix structure of DNA is also due to the hydrogen bonding between the nitrogenous bases of nucleotides.

Dipole-dipole

The attractive force that is present in molecules that has a permanent dipole or polar molecular is dipole-dipole interaction. And a dipole is a molecule that contains two types of charge they are positive and negative. So, in such a polar molecule, the positive side of one molecule gets attracted to the negative side of the other molecule. Therefore, the interaction will be present when there is proximity between two dipoles especially when they are in liquid form. This interaction is also present in molecules that a container has partial positive and negative charges. For example, in hydrogen chloride, 𝐻𝐶𝑙 dipole-dipole interaction is present when two molecules of a hydrogen chloride come closer. In which the chlorine which has a negative charge is attracted to the hydrogen which has a positive charge.

OpenStax,CNX Chem 10 01 DispForces, CC BY 4.0

Ion-ion interactions

The attractive force that is present in between ions of opposite charges is ion-ion interaction. And it will result in the formation of a bond that is ionic. For example in a saturated solution of sodium chloride, 𝑁𝑎𝐶𝑙 this interaction is present between the sodium ions, 𝑁𝑎+ and Chloride ions, 𝐶𝑙−. And this interaction will further result in the formation of sodium chloride crystals or salt. So, it is a common interaction in precipitation reactions.

Rhannosh, IonicBondingRH11, CC BY-SA 3.0

This interaction depends on the Coulomb force,

$$\mathrm{F\:\varpropto\:-\:\frac{Z_{1}Z_{2}}{r_{2}}}$$

Where 𝑍1 and 𝑍2 are charges and r are the distance between ions. So, when the distance between two eye answers increases the interaction will decrease.

London Dispersion Forces Examples

London disposition forces are present in nonpolar molecules. It is visible in molecules that don't contain any dipole moment. It is present in a molecule such as,

• Noble gases for example Neon, Argon, etc.

• Chlorine molecule, $\mathrm{Cl_{2}}$.

• Iodine molecule, $\mathrm{I_{2}}$.

• Bromine molecule, $\mathrm{Br_{2}}$.

• Hydrogen molecule, $\mathrm{H_{2}}$.

London Dispersion Forces Formula

Fritz London introduced a formula for the representation of a London force. By this formula, the interaction energy can be calculated. The formula is,

$$\mathrm{V\:=\:-\:\frac{C}{r^{6}}\:,\:Where\:C\:=\:\frac{2}{3}\alpha\:_{1}^{'}\:\alpha\:_{2}^{'}\:\frac{I_{1}\:I_{2}}{I_{1}\:+\:I_{2}}}$$

Where $\mathrm{\alpha^{'}_{1}\:and\:\alpha^{'}_{2}}$ are polarizability, and 𝐼1 𝑎𝑛𝑑 𝐼2are ionisation energy of molecules 1 and 2. And the term r is the distance between molecules.

Conclusion

There are several interactions present in different molecules. And is because of the electrostatic force. When a molecule or an atom that is nonpolar approaches each other there will be an attractive force operating on them. And this is the London force. It is the strongest force compared to other weak interactions. Noble gases show this type of interaction. Hydrogen bonding is also a type of interaction present in a hydrogen atom to an atom that has a lone pair of electrons. It is present in water molecules. Another interaction that exists between two ions is the ion-ion interaction. The Coulombic force of attraction determines the strength of ion-ion interaction.

FAQs

1. What is the order of melting and boiling point of noble gases based on London force?

As London forces strongly depend on the size of molecules. Larger molecules are strongly attracted to each other due to the London force, so to break their high amount of energy is needed. So, the order of melting and boiling point of noble gases is, $\mathrm{Xe\:>\:Kr\:>\:Ar\:>\:Ne\:>\:He}$

2. What are intermolecular and intramolecular hydrogen bonding?

The hydrogen bonding that is present between two different molecules is intermolecular hydrogen bonding. For example, in water molecules. And the hydrogen bonding that is present in the same molecule is Intramolecular hydrogen bonding. For example, in the o-nitrophenol molecule, intramolecular hydrogen bonding is present.

3. Which molecules have the dispersion force strongest?

In the Iodine molecule,$\mathrm{I_{2}}$ the dispersion force is stronger because of its large size.

4. What do you mean by dipole moment?

The property that arises in two different atoms present in a molecule because of its electronegativity difference. And it occurs when there is a separation of charge. For example, $\mathrm{HCl}$ has a dipole moment of 0.816 D.

5. What is ion-dipole interaction?

The interaction that arises between an ion with a neutral molecule is ion-dipole interaction. The interaction of an ion or polar molecule with a neutral or nonpolar molecule will induce polarity in that molecule. And is called ion-dipole interaction.