Intermolecular Forces: Definition, Types, Poles

Intermolecular Forces: The forces that form the basis of all interactions between different molecules are known as Intermolecular Forces. These forces are comparatively weaker than Intramolecular Forces (forces between atoms of one molecule). The strength of the intermolecular forces of attraction determines the type of interaction that will occur between two molecules, and the changes brought about by them due to these interactions.

Whenever Intermolecular forces of attraction examples are considered, a water molecule is the most common reference. The physical properties of matter are determined by intermolecular forces. Moreover, when we consider water, it is one of those substances that can occur in all three states – Solid, Liquid, and Gaseous state. The concept of intermolecular forces is important for the study of topics from both Physics and Chemistry. Read the complete article to know more.

What are Intermolecular Forces?

The forces of attraction or repulsion existing among the particles of atoms or molecules of a solid, liquid, or gaseous substance other than the electrostatic force that exists among the positively charged ions and forces that hold atoms of a molecule together, i.e., covalent bonds are called intermolecular forces. These differ from intramolecular forces examples which are certain types of covalent or ionic bonds.

Intermolecular forces are responsible for the structural features and physical properties of the substance. Intermolecular forces which exist within the same molecule or a polyatomic ion affect the chemical properties of the substance.

What are the 6 Types of Intermolecular Forces?

The different types of intermolecular forces come into existence due to the following types of interactions:

1. Dipole-dipole interactions
2. Dipole-induced dipole interactions
3. Ion-dipole interactions
4. Ion-induced dipole interactions
5. Dispersion forces
6. Hydrogen bonding

The intermolecular forces arising on account of dipole-dipole interaction, dipole induced dipole interaction, and dispersion forces are also referred to as van der Waals forces in honor of the Dutch scientist Johannes van der Waals.

Molecules Exhibiting the Strongest Intermolecular Force

The existence of the was studied by Keesom. Hence these forces are also called Keesom forces, and the effect is called the orientation effect.

Dipole-dipole interaction occurs among the polar molecules due to the permanent dipoles of a polar molecule. In a polar molecule, the positive pole of one molecule is attracted by the negative pole of the other molecule.

The magnitude of dipole-dipole forces in a different polar molecule can be predicted based on the electronegativity of the atom present in the molecule and the geometry of the molecule. The polarities of the molecule are usually expressed in terms of the dipole moment of the molecule. The dipole moment is expressed in Debye, which is represented by D.

Example: Dipole-dipole interaction present in the molecule of hydrogen chloride, which is polar \(\left( {{{\rm{H}}^{{\rm{\delta + }}}}{\rm{ – C}}{{\rm{l}}^{{\rm{\delta – }}}}} \right){\rm{.}}\) The chlorine being more electronegative has a partial negative charge \(\left( {{{\rm{\delta }}^{\rm{ – }}}} \right)\) while hydrogen has a partial positive charge \(\left( {{{\rm{\delta }}^{\rm{ + }}}} \right)\) as it is less electronegative than chlorine. Since only partial charges are involved, dipole-dipole interactions are weak. This further decreases with the increase in distance between the dipoles.

In stationary polar molecules, the dipole-dipole interaction energy between the molecules is proportional to the \(\frac{{\rm{1}}}{{{{\rm{r}}^{\rm{3}}}}}\) and that between the rotating molecule is proportional to \(\frac{{\rm{1}}}{{{{\rm{r}}^{\rm{6}}}}}{\rm{\;}}\) where \({\rm{r}}\) is the distance between the polar molecules.

How are Debye Forces Developed?

A polar molecule having a permanent dipole destroys a normal non-polar molecule and induces a dipole moment in it. This is known as dipole-induced dipole interactions. The force is developed due to interaction between a dipole, and the induced dipole is called Debye forces.

Debye forces come into existence when a polar molecule is brought closer to a non-polar molecule. The positive end of the polar molecule attracts the mobile electrons of the non-polar molecule, destroys it, and changes it into an induced dipole.

The positive end of the permanent dipole molecule can now add attract the displaced electron cloud of the induced dipole, and the two are held together by an electrostatic attraction. Debye forces are not affected by temperature. However, they depend upon the distance between the dipole and the induced dipole.

Example: Noble gases get polarised in the presence of polar molecules.

The existence of these forces was studied by Debye, and this effect is known as the induction effect.

What is Ion-Dipole Interaction?

The ion-dipole interaction involves the attraction between an ion (either a cation or an anion) and a polar molecule. The strength of ion-dipole interaction depends on the charge and size of the ion and also on the magnitude of dipole moment and size of the polar molecule. Since the charge density on cations is higher as compared to that on anion, cation attracts a dipole more strongly than an anion having the same charge but bigger size.

The hydration of ions is due to the ion-dipole interaction. When an ionic compound is dissolved in water, the ions attract water molecules which have a large dipole moment and get hydrated. Thus, water molecules act as a dielectric to keep the ions apart. The non-polar liquid such as carbon tetrachloride acts as a poor solvent for ionic compounds because they are unable to participate in ion-dipole interaction.

Example: When sodium chloride \(\left( {{\rm{NaCl}}} \right)\) is dissolved in water, the polar water molecules are attracted towards \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) ion as well as towards \({\rm{C}}{{\rm{l}}^{\rm{ – }}}\) ion. Due to the greater charge density on \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) this interaction usually stronger with \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) than with \({\rm{C}}{{\rm{l}}^{\rm{ – }}}\) having the same charge but bigger size. Further, \({\rm{CC}}{{\rm{l}}_{\rm{4}}}{\rm{,}}\) being non-polar, cannot interact with \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) and \({\rm{C}}{{\rm{l}}^{\rm{ – }}}\) ions. Hence, \({\rm{NaCl\;}}\) insoluble in \({\rm{CC}}{{\rm{l}}_{\rm{4}}}{\rm{.}}\)

How is Ion-Induced Dipole Interactions Developed?

A non-polar molecule may be polarised by the presence of an ion near it, i.e., it becomes an induced dipole. The interaction between them is called ion-induced dipole interactions. The strength of these interactions depends upon the charge on the ion and the ease with which the non-polar molecules get polarised. A cation polarises the molecule by the attraction of the electron cloud, whereas an ion does it by repulsion.

Example: in the presence of nitrate ion \(\left( {{\rm{NO}}_{\rm{3}}^{\rm{ – }}} \right){\rm{,}}\) iodine molecule \(\left( {{{\rm{I}}_{\rm{2}}}} \right){\rm{,}}\) which is nonpolar gets polarised as \({{\rm{I}}^{{\rm{\delta + }}}}{\rm{ – }}{{\rm{I}}^{{\rm{\delta – }}}}{\rm{.}}\)

Intermolecular Forces Attracting Non-polar Molecules or Monatomic Gases

The attractive forces come into existence due to instantaneous dipoles created in non-polar molecules like hydrogen \(\left( {{{\rm{H}}_{\rm{2}}}} \right){\rm{,\;}}\) oxygen \(\left( {{{\rm{O}}_{\rm{2}}}} \right){\rm{,\;}}\) chlorin \(\left( {{\rm{C}}{{\rm{l}}_{\rm{2}}}} \right){\rm{,}}\) iodine \(\left( {{{\rm{I}}_{\rm{2}}}} \right){\rm{,\;}}\) etc., and monatomic noble gases such as helium \(\left( {{\rm{He}}} \right){\rm{,}}\)neon\(\left( {{\rm{Ne}}} \right){\rm{,}}\) argon\(\left( {{\rm{Ar}}} \right){\rm{,}}\)xenon \(\left( {{\rm{Xe}}} \right){\rm{,}}\) etc., are called dispersion force or London force. It is also called instantaneous dipole interactions.

The existence of dispersion forces in such molecules is due to the development of an instantaneous or temporary dipole moment in them. Atoms and molecules are electrically symmetrical and, as such, do not possess any dipole moment.

However, any slight relative displacement of the nuclei or the electrons may develop an instantaneous or temporary dipole in them, and for a moment, they may act as a dipole. Such displacement is very common and constantly occurs in atoms and molecules. These displacements are temporary and random. Therefore, the molecule as a whole has no measurable dipole moment.

London forces are the weakest intermolecular forces. Their magnitude depends upon the following two factors:

1. Size/ Complexity of the molecules: larger or more complex are the molecules, the greater is the magnitude of London forces. This is because the larger electron clouds are easily distracted or polarised.
   Since the larger molecular size amounts to a larger molecular mass, it is often suggested that the magnitude of London forces increases with increasing molecular mass. Example: As we move down in group 18, the size of Noble gas increases, i.e., the size of electron cloud increases which gets more and more easily distorted. As a result, London’s force increases and so do their boiling points.

2. The geometry of the molecules: The shape of the molecules has a significant effect on the magnitude of London forces. For example, n-pentane and neopentane have the same molecular formula \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{12}}}}{\rm{,}}\) at the boiling point of n-pentane is about \({\rm{2}}{{\rm{7}}^{\rm{^\circ }}}\) higher than that of neo-pentane. The difference can be attributed to the different shapes of the two molecules, the n-pentane being a zig-zag chain, whereas neo-pentane is nearly spherical.

What is Hydrogen Bonding?

The weak attractive force which binds the partially positively charged hydrogen atom of one molecule, with the partially negatively charged atom of other molecules of a similar or different type, or with some other negative center of the same molecule, is referred to as hydrogen bond or hydrogen bonding.

Since hydrogen bonding arises because of dipole-dipole interactions, the magnitude of attractive forces depends on the inverse cube of the distance between the molecule \(\left( {\frac{{\rm{1}}}{{{{\rm{r}}^{\rm{3}}}}}} \right){\rm{.}}\)

Example: In the molecule of ammonia, \({\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\) the N atom is highly electronegative and acquires a partial negative charge due to the pulling of the shared pair. Therefore, in \({\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\) the H atom possesses a partial positive charge. Due to the presence of partial positive and negative charges, several molecules of \({\rm{N}}{{\rm{H}}_{\rm{3}}}\) linked together through hydrogen bonds.

How do Intermolecular Forces Affect Boiling Points?

The intermolecular forces are electrostatic and much weaker than the chemical forces. They exist in all the states of matter and play an important role in deciding several structural features and physical properties of matter.

An idea of the strength of intermolecular forces operating among the molecules of a substance can be obtained from the boiling point of the substance. The higher the boiling point, the greater is the magnitude of the intermolecular forces. Similarly, the melting points of substances increase with the increase in the strength of intermolecular forces.

Summary

In this article, you have understood different types of forces of interaction, i.e., intermolecular forces and their types in detail with suitable examples. This knowledge will help in studying the existence of different types of molecules.

FAQs on Intermolecular Forces

Following are some of the frequently asked questions on Intermolecular forces of attraction:

Q.1. How to determine intermolecular forces?
Ans. Intermolecular forces are determined based on the nature of the interacting molecule. For example, a non-polar molecule may be polarised by the presence of an ion near it, i.e., it becomes an induced dipole. The interaction between them is called ion-induced dipole interactions.

Q.2. How do intermolecular forces of attraction affect boiling point?
Ans. The higher the boiling point, the greater the magnitude of the intermolecular forces.

Q.3. Which are the strongest intermolecular forces?
Ans. Ion-dipole interaction is the strongest intermolecular force.

Q.4. What are the types of intermolecular forces?
Ans. The different types of intermolecular forces are dipole-dipole interactions, dipole-induced dipole interactions, ion-dipole interactions, ion-induced dipole interactions, dispersion forces, and hydrogen bonding.

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