Intermolecular force
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together. For example, the covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 present within HCl molecules is much stronger than the forces present between the neighbouring molecules, which exist when the molecules are sufficiently close to each other.

Intermolecular forces consist of four types:
  1. Dipole–dipole forces
  2. Ion–dipole forces
  3. Dipole-induced dipole
    Van der Waals force
    In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...

     force or Debye forces
  4. Instantaneous dipole-induced dipole forces or London dispersion forces.

London dispersion force

Otherwise known as quantum-induced instantaneous polarization or instantaneous dipole-induced dipole forces, the London dispersion force
London dispersion force
London dispersion forces is a type of force acting between atoms and molecules. They are part of the van der Waals forces...

 is caused by correlated movements of the electrons
Electronic correlation
Electronic correlation is the interaction between electrons in the electronic structure of a quantum system.- Atomic and molecular systems :...

 in interacting molecules. The electrons, which belong to different molecules, start "feeling" and avoiding each other at the short intermolecular distances, which is frequently described as formation of "instantaneous dipoles" that attract each other.

Debye (induced dipole) force

The induced dipole forces appear from the induction (also known as polarization), which is the attractive interaction between a permanent multipole on one molecule with an induced (by the former di/multi-pole) multipole on another. This interaction is called Debye force after Peter J.W. Debye.

The example of an induction-interaction between permanent dipole and induced dipole is HCl and Ar. In this system, Ar experiences a dipole as its electrons are attracted (to H side) or repelled (from Cl side) by HCl. This kind of interaction can be expected between any polar molecule and non-polar/symmetrical molecule. The induction-interaction force is far weaker than dipole-dipole interaction, however stronger than London force.They induce their properties in another atom

Dipole–dipole interactions

Dipole–dipole interactions are electrostatic interactions of permanent dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

s in molecules. These interactions tend to align the molecules to increase the attraction (reducing potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

). An example of a dipole–dipole interaction can be seen in hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 (HCl): The positive end of a polar molecule will attract the negative end of the other molecule and cause them to be arranged in a specific arrangement. Polar molecules have a net attraction between them. For example HCl and chloroform (CHCl3)

Keesom interactions (named after Willem Hendrik Keesom
Willem Hendrik Keesom
Willem Hendrik Keesom was a Dutch physicist who, in 1926, invented a method to freeze liquid helium.He also developed the first mathematical description of dipole-dipole interactions in 1921...

) are attractive interactions of dipoles that are Boltzmann-averaged
Boltzmann distribution
In chemistry, physics, and mathematics, the Boltzmann distribution is a certain distribution function or probability measure for the distribution of the states of a system. It underpins the concept of the canonical ensemble, providing its underlying distribution...

 over different rotational orientations of the dipoles. The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance.

Often, molecules have dipolar groups within them, but have no overall dipole moment. This occurs if there is symmetry within the molecule, causing the dipoles to cancel each other out. This occurs in molecules such as tetrachloromethane. Note that the dipole–dipole interaction between two atoms is usually zero, because atoms rarely carry a permanent dipole. See atomic dipoles.

Ion-dipole and ion-induced dipole forces

Ion-dipole and induced-dipole forces operate much like dipole-dipole and induced-dipole interactions. However, instead of only polar and non-polar molecules being involved, instead, ion interactions involve ions (as the name suggests). Ion-dipole and ion-induced dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole movement. Of course, H-bonds (a form of dipole-dipole) are still stronger than ion interactions.

An ion-dipole force consists of an ion and a polar molecule interacting. They align so that the positive and negative forces are next to one another, allowing for maximum attraction.

An ion-induced dipole force consists of an ion and a non-polar molecule interacting. Like a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud on the non-polar molecule.

Hydrogen bonding

A hydrogen bond is the attractive force between the lone pair of an electronegative atom and a hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 atom that is bonded to either nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, or fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

. The hydrogen bond is often described as a strong electrostatic dipole–dipole interaction. However, it also has some features of covalent bonding: It is directional, stronger than a van der Waals interaction
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...

, produces interatomic distances shorter than sum of van der Waals radii, and usually involves a limited number of interaction partners, which can be interpreted as a kind of valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...


Intermolecular hydrogen bonding is responsible for the high boiling point of water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 (100 °C) compared to the other group 16
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...

In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...

s that have no hydrogen bonds. Intramolecular hydrogen bonding is partly responsible for the secondary
Secondary structure
In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...

, tertiary
Tertiary structure
In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...

, and quaternary structure
Quaternary structure
In biochemistry, quaternary structure is the arrangement of multiple folded protein or coiling protein molecules in a multi-subunit complex.-Description and examples:...

s of protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s and nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

s. It also plays an important role in the structure of polymers, both synthetic and natural.

Relative strength of forces

Bond type Dissociation energy (kcal),
Covalent 400
Hydrogen bonds 12–16
Dipole–dipole 0.5–2
London (van der Waals) Forces <1

Note: this comparison is only approximate – the actual relative strengths will vary depending on the molecules involved.

Quantum mechanical theories

See also

  • Coomber's relationship
    Coomber's relationship
    Coomber's relationship can be used to describe how the internal pressure and dielectric constant of a non-polar liquid are related.As p_i=\left_T\,, which defines the internal pressure of a liquid, it can be found that:...

  • Force field
    Force field (chemistry)
    In the context of molecular modeling, a force field refers to the form and parameters of mathematical functions used to describe the potential energy of a system of particles . Force field functions and parameter sets are derived from both experimental work and high-level quantum mechanical...

  • Hydrophobic effect
    Hydrophobic effect
    The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in aqueous solution and exclude water molecules. The name, literally meaning "water-fearing," describes the segregation and apparent repulsion between water and nonpolar substances...

  • Intramolecular force
    Intramolecular force
    An intramolecular force is any force that holds together the atoms making up a molecule or compound.-Types of intramolecular force:There are three main types of intramolecular force:*Ionic*Covalent*Metallic...

  • Molecular solid
    Molecular solid
    Molecular solid is a solid composed of molecules held together by the van der Waals forces. Because these dipole forces are weaker than covalent or ionic bonds, molecular solids are soft and have relatively low melting temperature. Pure molecular solids are electrical insulators but they can be...

  • Polymer
    A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

  • Quantum chemistry computer programs
    Quantum chemistry computer programs
    Quantum chemistry computer programs are used in computational chemistry to implement the methods of quantum chemistry. Most include the Hartree–Fock and some post-Hartree–Fock methods. They may also include density functional theory , molecular mechanics or semi-empirical quantum...

  • Software for molecular mechanics modeling

External links

Software for calculation of intermolecular forces
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