Van Der Waal forces are weak and short-range electrostatic attractive forces between uncharged molecules. It arises from the interaction between permanent or transient electric dipole moments.
It is named after the Dutch scientist Johannes Diderik Van Der Waals. In 1873 he first discovered these forces while developing a theory on the characteristics of real gases. Van Der Waals forces hold the solids that have a low melting point. On the other hand, the objects that are held together by metallic, covalent and ionic bonds have a high boiling point.
Van der Waal forces and Casimir have the same origin, emerging from the quantum interactions along with zero-point energy.
Electrons are distributed throughout an atom or molecule. Moreover, electrons are always moving. At any moment there can be uneven distributions of electrons. This causes the formation of a temporary dipole. This dipole can cause another temporary dipole to form in a neighbouring atom. It results in a domino effect of temporary dipoles and the atoms or molecules becoming attracted to each other. It is called Vander wall force.
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Fig1: Vanderwall force diagram
Following are the characteristics of Van Der Waal forces.
These forces are additive.
They are weaker than ionic, covalent bonds.
Van Der Waals forces are not directional.
They work in a short range. The interaction increases when molecules are closer.
These forces are independent of temperature. The only exception is dipole-dipole interaction.
The strength of these forces usually ranges between 0.4 kJ/mole and 4 KJ/mole. Moreover, they act over a distance of less than O.4 nm. However, the net effect of these forces is repulsive because electron clouds repel.
Due to the Pauli Exclusion Principle, a negative component does not let the molecules collapse.
Keesom interaction is another significant component of this force. This attractive or repulsive interaction takes place between dipoles, permanent charges, multipoles and quadrupoles.
Polarisation or induction occurs in this force. It is an attractive force acts upon a permanent polarity of one molecule and on another one’s induced polarity. It is called Debye Force.
London Dispersion force also occurs because of instantaneous polarisation. Non-polar molecules also experience this force.
Primarily there are three types of Van Der Waal forces. Following are the list of those.
Dipole is a polar molecule in which a certain distance separates partial positive and partial negative charges. It has a permanent dipolar moment. Dipole-dipole interaction results in generating an attractive force between neighbouring polar molecules. It results from the electrostatic force of attraction.
For example, a hydrogen molecule consists of a partially positive charged hydrogen atom and a negatively charged chlorine atom. When there are a collection of HCl molecules, they will rearrange themselves in such a way where oppositely charged poles of surrounding molecules are near to each other.
These Van Der Waals interactions are stronger than Dispersion forces but weaker than ion-ion interactions.
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Fig2: London dispersion forces in Helium
The molecular forces between non-polar molecules are called London dispersion forces. These forces depend on the polarizability of a molecule or an atom. London forces are attractive forces arising due to instantaneous dipoles. The strength of Van Der Waal Dispersion forces increases with the size, shape, molar mass and number of electrons in an atom or a molecule. It is considered as the weakest of all intermolecular forces.
Dispersion forces are present in all molecules, but non-polar molecules have only these forces. It is called London forces as in 1930 Fritz London first discovered their existence. These forces occur between non-polar molecules and between atoms. It happens due to the motion of the electrons.
For example, these forces are found in halogens (F2 and I2), noble gases and some non-polar molecules like methane and carbon dioxide. Also, these forces are stronger in the molecules which are not compact and arranged in a long chain of elements. It is so because the electrons can be displaced easily due to the weaker attraction between protons and electrons within the nucleus.
These Van der Waals forces of attraction act between the polar molecules containing permanent dipole and molecules lacking permanent dipoles. Polar molecule’s permanent dipole infuses dipole on electrically neutral molecules by altering its electronic cloud. In the process, an induced dipole is created in another molecule. In this force, the interaction energy is proportional to 1/r6. Also, r is the distance between the molecules.
When a polar molecule with non-zero dipole moment approaches a non-polar molecule within a distance of 10-9 to 10-10 m, the polar molecule induces a temporary dipole in the non-polar molecule. It happens due to the distortion and separation of charges.
Thus Van Der Waals bond largely configure the characteristics of molecules.
Do It Yourself: Make a chart and write down the differences between Van Der Waals and hydrogen bonding.
The strength of these forces mainly depends on the following factors.
Size of the molecule- If you look at the periodic table, you will see that the boiling point of noble gases increases, as you go down. It happens because the numbers of electrons increase along with the radius of the atoms. Thereby, if the electrons are more in number, the distance will be more, and there will be more possibility of temporary dipoles. Owing to this reason, bigger molecules possess higher boiling points than the smaller ones. For example, Xenon atoms have higher boiling points than the Neon ones.
Shape of the molecule- Molecules shapes also plays a crucial role in determining the strength of these forces. Thin long molecules are capable of developing more temporary dipoles than fat short ones, carrying an equal number of electrons. Also, these thin molecules can stay close to each other; therefore, the attraction between them is most effective. For example, butane and 2-methyl propane have the same molecular formula. However, the boiling point of butane is higher because they are arranged in a longer chain than the other one.
These are the major factors affecting van der Waal forces.
This equation is needed to calculate the actual values of some special cases, for example, real gases.
(P+n2a/V2) (V-nb)= nRT
V refers to the gas volume in moles n. Value of a specific gas is a. P refers to the pressure measured. Also, b is a variable that denotes the eliminated volume per mole. R is a known constant = 0.08206 L atm mol-1 K-1. Finally, T represents temperature.
The applications of Van Der Waals forces are many. Some of them are discussed below.
These forces play a vital role in folding and stabilising protein structure.
They also help in bonding graphenes in graphite.
Takes part in polymer formation, nanotechnology, supermolecular chemistry, etc.
Gecko Lizards, with the help of these forces, climb the walls. Similarly, some other animals walk on water due to this.
These are a few Van Der Waals forces examples that we can see in daily life.
Vander Waals forces are a kind of distance-dependent forces between molecules and atoms.
Often this term is utilised to encompass all types of intermolecular forces. However, most scientists have associated it with Keesom, Debye and London Dispersion forces.
These forces are the weakest of all the intermolecular forces. However, they contribute to a major property of molecules.
Now It Is Time For A Short Quiz On Vanderwall Force!
1. Which one of the following molecules has dipole-dipole interaction?
2. What interaction can occur between Xenon molecules and water?
Dipole-induced dipole interaction.
Instantaneous dipole-induced dipole interaction.
Answers: 1-d), 2-b).
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1. Which Is The Weakest Bond In Chemistry?
The weakest bond is chemistry is the Van Der Waal force interaction. However, the interaction can be very strong when this force works between two objects with high intensity.
2. What Is Cause Of Van Der Waals Forces?
Mainly the attraction and repulsion between molecules, atoms, surfaces and also other molecular forces cause Van Der Waals forces.
3. How Many Types Of Forces Are There In Van Der Waal Forces?
Primarily there are two types of Van Der Waal Forces- dipole-dipole forces, dispersion force, ion-dipole force.
4. What Can Break Van Der Waal Force?
The molecules get excited and started to move due to heat. Therefore, if the movements increase, Van Der Waal force does not work, and the molecules separate from each other. Also in water, the hydrogen bonds break if external energy is infused in the system.