London Dispersion Forces| The Weakest Ones

London dispersion forces are weak intermolecular forces between non-polar molecules, commonly known as instantaneous dipoles-induced dipoles. These forces are named after the German physicist, Fritz London (1900-1954).

Electrons are considered to be symmetrically distributed around the nucleus. However, the electrons are also in continuous motion. At times, this movement allows the electrons to be concentrated in a certain region of space around the nucleus. 

This fluctuation in the electron cloud density creates dipoles in otherwise non-polar molecules. As the electrons are negatively charged, the region where the electron density increases, acquires a partial negative charge, along with a corresponding partial positive charge where the electron density is reduced. The fluctuation is in relation to the normal, symmetric electronic cloud distribution.

The dipole produced as a result of the fluctuation is referred to as the instantaneous or temporary dipole. When such a charge distribution is created, it will induce further dipoles in the neighboring polarizable molecules.

The partial negative ends of this dipole will push the electron density in the neighboring molecule away, inducing dipoles in the other molecule. 

On the contrary, the partial positive end, being somewhat electron deficient, will attract electrons from the adjacent molecule, inducing dipole in that molecule as well.

With the opposing partial charges adjacent, there form electrostatic forces of attraction between the opposite charges, holding the molecules together. These forces are known as the London dispersion forces.

London Dispersion Force

Also known as the “temporary dipole-induced dipole”, “instantaneous dipole-induced dipoles”, or “fluctuating dipole-induced dipole”, or simply “London” or “dispersion” forces, the interactions are weak and short-lived.

London dispersion force is one of the three constituents of Van der Waals forces of attraction, with the other two being the Keesom and the Debye forces.

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Van der Waals forces (or intermolecular forces) also include the electrostatic repulsion component that prevents the collapse of molecules into one another.

Properties of London Dispersion Forces

London dispersion forces in general, are the weakest type of intermolecular forces. The stronger ones are dipole-dipole interactions, hydrogen bonding, and ion-dipole interactions, in order of increasing strength.

These intermolecular forces affect various properties of a substance, such as the melting and boiling points. Stronger intermolecular forces correspond to higher melting and boiling points.

It implies that non-polar substances, possessing London dispersion forces, will have lower melting and boiling points than polar substances in general.

A simple example of a non-polar substance is helium gas. In a helium atom, the electron cloud is uniformly distributed in a spherical region around the nucleus. Yet, at any given instant, the actual location of its electrons relative to the nucleus can create an instantaneous dipole in the atom. 

In turn, this temporary dipole influences the distribution of electrons in neighboring helium atoms, producing induced dipoles in those atoms. 

The attraction between the opposite charges of the instantaneous dipole and the induced dipole is the London dispersion force.

Factors Affecting London Dispersion Forces

The factors that influence the strength of London dispersion forces are as follows:

  1. Molecular size
  2. Polarizability
  3. Number of atoms in the molecule

Molecular Size

The strength of intermolecular forces depends primarily on the atomic or molecular size. With the increase in the size of a molecule, the dispersion of the electron cloud becomes easier. So the intermolecular forces are more pronounced in larger molecules.

Noble gases are generally inert and exist as monoatomic gases. The boiling point of these gases increases down the group, from helium to radon. This is because of the increasing atomic size of atoms, and hence the ease of dispersion.

Polarizability

Another factor that influences the strength of London dispersion forces is the polarizability of the atoms. Polarizability refers to the ease of distortion of the electron charge density around the nucleus. 

Larger atoms have more electrons and therefore, a bigger electron cloud than smaller atoms. Since the electrons are farther away from the nucleus in the larger atoms, the valence electrons are more loosely bound and can move towards another atom easily as compared to the electrons that are comparatively tightly bound (in smaller atoms).

In halogens (group 7), polarizability increases down the group (F < Cl < Br < I) along with the increasing size of the atoms. Dipoles are created more easily, and as a result, the dispersion forces are stronger between the molecules. 

This is the reason why the first member of the halogens, fluorine, is a gas at room temperature. The second member, chlorine, is also a gas but is liquified easily as compared to fluorine. Bromine, on the other hand, is a liquid, while, iodine is a solid at room temperature.

Number of Atoms in a Molecule

The third factor is the number of atoms in the molecule, and the resulting higher surface area. The higher the number of atoms in a molecule, the larger the molecule will be. It can be easily polarized and can form intermolecular forces over a greater surface area, resulting in an overall stronger intermolecular force of attraction.

This is made evident by comparing the boiling points of pure hydrocarbons at any fixed pressure. The first four alkanes, methane, ethane, propane, and butane, are all gases at 1 atm pressure. Their boiling points increase successively from -161.6°C to -1°C. Pentane and onwards alkanes are liquids or solids at RTP. 

Boiling Point of Alkanes

This shows that long-chain molecules have stronger intermolecular forces of attraction.

London Formula

The strength of London dispersion interaction depends on the polarizability of the initial molecule as the instantaneous dipole moment depends on the loose control of the nuclear charge on the valence electrons.

The strength of the dispersion interaction also depends on the polarizability of the second molecule, as to the ease with which its electron cloud can be distorted and a dipole can be induced.

An approximation of the energy involved in the interaction is given by the London formula:

V = – C / r6, where C = (2/3) 𝛼1’𝛼2’ (I1I2 / I1 + I2)

where;

  • I1 and I2 are the ionization energies of the two molecules

Concepts Berg

What causes London dispersion forces?

Fluctuations in the electron density relative to the nucleus produce instantaneous dipoles. These temporary dipoles induce further dipoles in the neighboring molecules. The electrostatic attraction between these instantaneous and induced dipoles is termed as London dispersion force.

Is HF London dispersion?

London dispersion force is the main force of attraction between non-polar molecules. HF, being polar, would possess dipole-dipole interactions. But since fluorine is a highly electronegative element, a hydrogen bond is formed instead, which is a stronger type of dipole-dipole interaction.

Do all molecules have London dispersion forces?

London dispersion force is the main force of attraction in non-polar molecules. However, polar molecules can also interact through instantaneous dipoles. The only difference is that the time average does not make the dipole disappear (as in non-polar molecules), but rather corresponds to the permanent dipole.

How do you identify London forces?

London dispersion forces are the reason non-polar gaseous substances can change to liquid and solid states provided the temperature is low enough. These forces are present in non-polar and polar molecules, although the dispersion forces are the primary intermolecular forces in non-polar substances. Polar molecules, on the other hand, possess stronger dipole-dipole interactions.

Do gases have London dispersion forces?

It is the strength of intermolecular forces that decide whether a substance exists as a solid, liquid, or gas at room temperature. In general, London dispersion forces are the weakest type of intermolecular forces. So it is highly likely that a substance existing as a gas at room temperature possesses London dispersion forces.

How do London dispersion forces form between polar molecules?

At any given time, the electron density flickers relative to the nuclei and produces an instantaneous dipole. This dipole can induce dipoles further and form London dispersion forces with them. This happens the same way as in non-polar molecules. The sole difference is that in polar molecules, the dipole does not vanish in the time average.

What is another name for London dispersion forces?

London dispersion forces are commonly known as “Instantaneous dipoles-induced dipoles”. They can also be referred to as “Temporary dipole-induced dipole”, or simply as “London” or “Dispersion” forces.

How do London dispersion forces appear at low temperatures?

At low temperatures, the molecules possess lower kinetic energy. The molecules slow down and come close to each other; more intermolecular forces begin to form between them. At certain low temperatures, the gases change into a liquid state and then to a solid state.

Do metal atoms have London-dispersion force?

Since metal atoms too have electrons, they possess London dispersion forces as well. However, the dispersion force would be less significant because metal atoms/ions are held together by a much stronger electrostatic force known as the metallic bond. The metallic bond is often defined as fixed positive charges in a sea of delocalized electrons.

On the contrary, London dispersion forces are more significant in molecules, especially the non-polar ones.

Is induced dipole the same as London dispersion?

London dispersion forces can also be called Instantaneous/temporary dipoles-induced dipoles. However, permanent dipole-induced dipoles are termed as Debye forces instead.

What exactly is the London dispersion force in chemistry?

London dispersion force is the weakest type of intermolecular forces of attraction. This dispersion force is the main force of attraction between non-polar molecules, and its purpose is to hold the molecules together.

References

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