Intermolecular Forces – The Forces of Attraction

Definition of Intermolecular Forces

The attractive forces exist between individual particles i.e. atoms, molecules, and ions.


It is very important to realize that the attraction between the particles is much weaker than the attraction between atoms within a molecule.

  • In a molecule of HCl, there is a covalent bond between H and Cl which is because of the mutual sharing of electrons.
  • Both atoms satisfy their outermost shells and it is their firm needs to stay together, hence this bond is very strong.
  • HCl molecules in the nearby attract each other, but the forces of attraction are weak.
  • These forces are thought to exist between all types of atoms and molecules when they are sufficiently near each other.
  • Such intermolecular forces are called van der Waals forces and they have nothing to do with the valence electrons.
  • These intermolecular forces bring the particles close together and give specific physical properties to the substances in gaseous, liquid, and solid states.
Types of Intermolecular Forces

The 4 types of such forces are discussed here:

  1. Dipole-dipole forces
  2. Ion-dipole forces
  3. Dipole-induced dipole forces
  4. Instantaneous dipole-induced dipole forces or London dispersion forces
Dipole-dipole Forces

“The positive end of one molecule attracts the negative end of the other particle and these electrostatic forces of attraction are called dipole-dipole forces.”

In the case of the HCl molecule, both atoms differ in electronegativity. Chlorine is more electronegative and develops a partial negative charge and hydrogen develops a partial positive charge. So, whenever the particles are close to each other, they tend to line up. Thus, attracts the opposite ends. However, thermal energy causes the particles not to have a perfect alignment.

Anyways, there is a net attraction between the polar molecules. These forces are called dipole-dipole forces and they are roughly one percent as effective as a covalent bond. The strength of these forces depends upon the electronegativity difference between the bonded atoms and the distance between the molecules.

The distance between molecules in the gaseous phase is greater so these forces are extremely weak in this phase. In liquids, these forces are fairly strong.

The greater the strength of these dipole-dipole forces, the higher the values of thermodynamic parameters like melting points, boiling points, the heat of vaporization, and heats of sublimation.


The examples of the particles which reveal dipole-dipole attractions are numerous. Two of these are HCl and CHCl3 (chloroform).


Ion-dipole forces

An ion-dipole force is an attractive force that arises from the electrostatic attraction in between an ion and a neutral particle that has a dipole. Most typically present in solutions. A positive ion (cation) draws in the partially negative end of a neutral polar molecule.


Dipole-induced Dipole Forces

In some cases, we have a mixture of substances including polar and non-polar molecules. The positive end of the polar particle attracts the mobile electrons of the neighboring non-polar molecule. In this way, polarity is caused in the non-polar molecules, and both molecules become dipoles. These forces are called dipole-induced dipole forces or Debye forces.



Instantaneous Dipole-induced Dipole Forces or London Dispersion Forces

The momentary force of attraction created in between the instant dipole and the induced dipole is called immediate dipole-induced dipole interaction or London forces.

The forces of attraction present amongst the non-polar molecules like helium, neon, argon, chlorine, and methane require unique attention because under typical conditions such particles do not have dipoles. We know that helium gas can be liquified under proper conditions. In other words, forces of attraction run among the atoms of helium which trigger them to cling together in the liquid state.

  • A German physicist Fritz London in 1930 offered a simple description of these weak appealing forces in between non-polar particles.
Further Reading:  Occurrence, Properties, Uses and Isotopes of Argon


In helium gas, the electrons of one atom influence the moving electrons of the other atom. Electrons fend off each other and they tend to stay as far apart -as possible. When the electrons of one atom come close to the electron of another atom, they are pushed far from each other. In this way, a short-term dipole is produced in the atom.

The result is that, at any moment, the electron density of the atom disappears balanced. It has a more negative charge on one side than on the other. At that particular moment, the helium atom becomes a dipole. This is called an immediate dipole.


This rapid dipole then disrupts the electronic cloud of the other nearby atom. So, a dipole is induced in the 2nd atom. This is called an induced dipole.

It is a very short-term attraction since the electrons keep moving. This motion of electrons causes the dipoles to disappear as rapidly as they are formed. Anyhow, a minute later on, the dipoles will appear in a different orientation, and once again weak attractions are established.


London forces exist in all kinds of particles whether polar or non-polar, but they are very significant for non-polar particles like Cl2, H2, and noble gases (helium, neon, etc.).

Factors Impacting the London Forces
  • London forces are weaker than dipole-dipole interactions. The strength of these forces depends upon the size of the electronic cloud of the atom or molecules. When the size of the atom or molecule is big then the dispersion becomes easy and these forces end up being more prominent. The elements of the zero group in the periodic table are all mono-atomic gases. They don’t make covalent bonds with other atoms since their outermost shells are complete. Their boiling points increase down the group from helium to radon.
  • The atomic number increases down the group and the outer electrons move away from the nuclei. The dispersion of the electronic clouds ends up being increasingly easier. So, the polarizability of these atoms goes on increasing.
  • Polarizability is the quantitative measurement of the degree to which the electron cloud can be polarized or distorted. When we say that a species (atom, molecule, or ion) is polarized, it suggests that short-term poles are developed. This is possible if the electronic cloud can be interrupted or distorted. This increased distortion of the electronic cloud produces stronger London forces and for this reason, the boiling points are increased down the group.
  • Similarly, the boiling points of halogens in group VII-An also increase from fluorine to iodine. All the halogens are nonpolar diatomic molecules, but there is a huge difference in their physical states at room temperature. Fluorine is a gas and boils at -188.1 ° C, while iodine is a solid at room temperature which boils at +184.4 ° C. The polarizability of the iodine molecules is much greater than that of fluorine.
  • Another essential factor that affects the strength of London forces is the number of atoms in a non-polar molecule. The greater the number of atoms in a molecule, the higher is its polarizability. Let us talk about the boiling points of saturated hydrocarbons. These hydrocarbons have a chain of carbon atoms linked with hydrogen atoms. Compare the length of the chain for C2H6 and C6H14. They have the boiling points – 88.6 ° C and 68.7 °C, respectively. This means that the molecule with a large chain length experiences a stronger attractive force. The factor is that longer molecules have more places along its length where they can be attracted to other particles. It is very intriguing to understand that with the increasing molecular mass of these hydrocarbons, they change from gaseous to liquid and after that finally end up being solids.
Further Reading:  Alkanes [Properties, Structure, Reactivity, and Uses of Alkanes]

Multiple Choice Questions (MCQs) with Answers

  1. What do intermolecular forces refer to?
    • a) Forces within a molecule
    • b) Forces between atoms and molecules
    • c) Forces within an atom
    • d) Forces between electrons

    Answer: b

  2. How do intermolecular forces compare to intramolecular forces?
    • a) Weaker
    • b) Stronger
    • c) Equal
    • d) Non-existent

    Answer: a

  3. Which type of bond within a molecule is considered strong?
    • a) Ionic bond
    • b) Metallic bond
    • c) Covalent bond
    • d) Dipole-dipole bond

    Answer: c

  4. What term is used for the intermolecular forces called van der Waals forces?
    • a) Ionic forces
    • b) Covalent forces
    • c) London forces
    • d) Dipole-dipole forces

    Answer: c

  5. Which factor impacts the strength of dipole-dipole forces?
    • a) Electronegativity difference
    • b) Atomic mass
    • c) Distance between molecules
    • d) Both a and c

    Answer: d

  6. In dipole-dipole forces, what causes the net attraction between polar molecules?
    • a) Perfect alignment
    • b) Thermal energy
    • c) Covalent bonds
    • d) Ionic bonds

    Answer: b

  7. Ion-dipole forces most commonly occur in which type of solutions?
    • a) Gaseous solutions
    • b) Liquid solutions
    • c) Solid solutions
    • d) Aqueous solutions

    Answer: d

  8. What is the primary characteristic of dipole-induced dipole forces?
    • a) Interaction between two dipoles
    • b) Induction of polarity in non-polar molecules
    • c) Covalent bonding
    • d) Metallic bonding

    Answer: b

  9. Which scientist provided a simple explanation for London dispersion forces?
    • a) Albert Einstein
    • b) Fritz London
    • c) Marie Curie
    • d) Max Planck

    Answer: b

  10. What is an immediate dipole in London dispersion forces?
  • a) A permanent dipole
  • b) A temporary dipole
  • c) A strong dipole
  • d) A covalent dipole

Answer: b

  1. What is the significance of London forces for non-polar particles?
  • a) They enhance covalent bonding
  • b) They induce polarity
  • c) They provide weak attractive forces
  • d) They create ionic bonds

Answer: c

  1. What factor determines the strength of London forces in a molecule?
  • a) Electronegativity
  • b) Size of the electronic cloud
  • c) Covalent radius
  • d) Atomic number

Answer: b

  1. How does polarizability affect London forces?
  • a) Decreases dispersion
  • b) Increases dispersion
  • c) Has no effect on dispersion
  • d) Converts dispersion to covalent bonding

Answer: b

  1. What is polarizability a measure of?
  • a) Covalent bonding strength
  • b) Ionic bonding strength
  • c) Electron cloud distortion
  • d) Dipole-dipole interaction

Answer: c

  1. Why do boiling points increase down the zero group in the periodic table?
  • a) Decreased atomic number
  • b) Increased atomic number
  • c) Decreased polarizability
  • d) Increased polarizability

Answer: d

  1. Why do the boiling points of halogens increase from fluorine to iodine?
  • a) Decreased polarizability
  • b) Increased polarizability
  • c) Decreased atomic number
  • d) Increased atomic number

Answer: b

  1. What is the role of the number of atoms in a non-polar molecule in London forces?
  • a) Increases dispersion
  • b) Decreases dispersion
  • c) Has no effect on dispersion
  • d) Converts dispersion to covalent bonding

Answer: a

  1. How do longer hydrocarbon chains affect London forces?
  • a) Decrease in strength
  • b) Increase in strength
  • c) No impact on strength
  • d) Conversion to covalent bonding

Answer: b

  1. What is the boiling point trend for saturated hydrocarbons with longer chain lengths?
  • a) Decrease
  • b) Increase
  • c) Remain constant
  • d) Convert to solids

Answer: b

  1. In intermolecular forces, what happens to longer molecules in terms of attraction?
  • a) Attraction decreases
  • b) Attraction increases
  • c) No change in attraction
  • d) Attraction converts to repulsion
Further Reading:  Charles's Law [Formula, Experimental Verification, Graphical Explanation]

Answer: b

  1. Which intermolecular force is significantly important for non-polar particles like Cl2 and H2?
  • a) Dipole-dipole forces
  • b) Ion-dipole forces
  • c) London forces
  • d) Dipole-induced dipole forces

Answer: c

  1. What is the primary characteristic of instantaneous dipole-induced dipole forces?
  • a) Strong and permanent
  • b) Weak and temporary
  • c) Covalent bonding
  • d) Ionic bonding

Answer: b

  1. What factor makes the London forces weaker than dipole-dipole interactions?
  • a) Electronegativity
  • b) Atomic mass
  • c) Distance between molecules
  • d) Polarizability

Answer: b


Frequently Asked Questions (FAQs) – Intermolecular Forces

1. What are intermolecular forces?

  • Intermolecular forces refer to the attractive forces between individual particles such as atoms, molecules, and ions.

2. How do intermolecular forces compare to intramolecular forces?

  • Intermolecular forces are much weaker than the intramolecular forces (e.g., covalent bonds) within a molecule.

3. What is the role of van der Waals forces in intermolecular interactions?

  • Van der Waals forces are intermolecular forces present between atoms and molecules when they are in close proximity.

4. How do dipole-dipole forces work?

  • Dipole-dipole forces occur when the positive end of one molecule attracts the negative end of another, creating electrostatic forces of attraction.

5. Can you provide examples of particles exhibiting dipole-dipole attractions?

  • Examples include HCl and CHCl3 (chloroform).

6. What characterizes ion-dipole forces, and where are they commonly found?

  • Ion-dipole forces arise from the electrostatic attraction between an ion and a neutral particle with a dipole. They are commonly found in solutions.

7. Explain dipole-induced dipole forces.

  • These forces occur in mixtures of polar and non-polar molecules, where the positive end of a polar particle attracts the electrons of a neighboring non-polar molecule.

8. What are London dispersion forces, and why are they significant for non-polar particles?

  • London forces are weak attractions present among non-polar particles, such as helium and neon. They become significant under specific conditions.

9. Who provided a simple explanation of London dispersion forces?

  • German physicist Fritz London offered a simple description of these weak attractive forces in 1930.

Summary: Intermolecular Forces – The Forces of Attraction

This tutorial comprehensively explores intermolecular forces, shedding light on the fundamental aspects and various types that influence the physical properties of substances in gaseous, liquid, and solid states.

Key Points:

  1. Definition of Intermolecular Forces:
    • Attractive forces exist between individual particles, including atoms, molecules, and ions.
  2. Van der Waals Forces:
    • Weaker than intramolecular forces, such as covalent bonds within a molecule.
    • Present between particles in close proximity, known as van der Waals forces.
  3. Types of Intermolecular Forces:
    • Dipole-dipole forces, ion-dipole forces, dipole-induced dipole forces, and London dispersion forces.
  4. Dipole-dipole Forces:
    • Electrostatic forces where the positive end of one molecule attracts the negative end of another.
    • Examples include HCl and CHCl3 (chloroform).
  5. Ion-dipole Forces:
    • Arise from electrostatic attraction between an ion and a neutral polar molecule.
    • Commonly found in solutions.
  6. Dipole-induced Dipole Forces:
    • Occur in mixtures of polar and non-polar molecules, inducing polarity in non-polar molecules.
  7. London Dispersion Forces:
    • Weak forces among non-polar molecules like helium, neon, argon, etc.
    • Fritz London explained these forces in 1930.
  8. Factors Impacting London Forces:
    • Weaker than dipole-dipole interactions, influenced by the size of the electronic cloud.
    • Boiling points increase down the periodic table, influenced by atomic size and polarizability.
    • Number of atoms in a non-polar molecule affects polarizability.
  9. Boiling Points of Halogens:
    • Increase from fluorine to iodine due to differences in polarizability.
  10. Effect of Molecular Mass on London Forces:
    • Longer hydrocarbon chains experience stronger attractive forces, transitioning from gaseous to solid with increasing molecular mass.

This tutorial provides a comprehensive understanding of intermolecular forces, their types, and the impact on substance properties, offering valuable insights into the underlying principles governing molecular interactions.