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 Intermolecular Forces 

What are Intermolecular Forces?

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions ). These forces are weak compared to the intramolecular forces, such as the covalent or ionic bonds between atoms in a molecule. For example, the covalent bond present within a hydrogen chloride (HCl) molecule is much stronger than any bonds it may form with neighboring molecules

Types of Attractive Intermolecular Forces


 * 1) Dipole-dipole forces: electrostatic interactions of permanent dipoles in molecules; includes hydrogen bonding.
 * 2) Ion-dipole forces: electrostatic interaction involving a partially charged dipole of one molecule and a fully charged ion.
 * 3) Instantaneous dipole-induced dipole forces or London dispersion forces: forces caused by correlated movements of the electrons in interacting molecules, which are the weakest of intermolecular forces and are categorized as van der Waals forces

Dipole-Dipole Attractions
Dipole–dipole interactions are a type of intermolecular attraction—attractions between two molecules. Dipole-dipole interactions are electrostatic interactions between the permanent dipoles of different molecules. These interactions align the molecules to increase the attraction.

An electric monopole is a single charge, while a dipole is two opposite charges closely spaced to each other. Molecules that contain dipoles are called polar molecules and are very abundant in nature. For example, a water molecule (H2O) has a large permanent electric dipole moment. Its positive and negative charges are not centered at the same point; it behaves like a few equal and opposite charges separated by a small distance. These dipole-dipole attractions give water many of its properties, including its high surface tension.

Uneven Distribution of Electrons
The permanent dipole in water is caused by oxygen ‘s tendency to draw electrons to itself (i.e. oxygen is more electronegative than hydrogen). The 10 electrons of a water molecule are found more regularly near the oxygen atom’s nucleus, which contains 8 protons. As a result, oxygen has a slight negative charge (δ-). Because oxygen is so electronegative, the electrons are found less regularly around the nucleus of the hydrogen atoms, which each only have one proton. As a result, hydrogen has a slight positive charge (δ+).

Examples of Dipole-Dipole Interactions
Another example of a dipole–dipole interaction can be seen in hydrogen chloride (HCl): the relatively positive end of a polar molecule will attract the relatively negative end of another HCl molecule. The interaction between the two dipoles is an attraction rather than full bond because no electrons are shared between the two molecules.

Hydrogen Bonds
Hydrogen bonds are a type of dipole-dipole interactions that occur between hydrogen and either nitrogen, fluorine, or oxygen. Hydrogen bonds are incredibly important in biology, because hydrogen bonds keep the DNA bases paired together, helping DNA maintain its unique structure.

Ion-Dipole Force
The ion-dipole force is an intermolecular attraction between an ion and a polar molecule.

Ion-Dipole Force
Ion-dipole and ion-induced dipole forces operate much like dipole-dipole and induced dipole-dipole interactions. However, ion-dipole forces involve ions instead of solely polar molecules. Ion-dipole forces are stronger than dipole interactions because the charge of any ion is much greater than the charge of a dipole; the strength of the ion-dipole force is proportionate to ion charge. Ion-dipole bonding is also stronger than hydrogen bonding. An ion-dipole force consists of an ion and a polar molecule aligning so that the positive and negative charges are next to one another, allowing for maximum attraction.

Ion-dipole forces are generated between polar water molecules and a sodium ion. The oxygen atom in the water molecule has a slight negative charge and is attracted to the positive sodium ion. These intermolecular ion-dipole forces are much weaker than covalent or ionic bonds

Ion-Induced Dipole Force
An ion-induced dipole force occurs when an ion interacts with a non-polar molecule. Like a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud in the non-polar molecule, causing a temporary partial charge. The temporary partially charged dipole and the ion are attracted to each other and form a fleeting interaction.

Dispersion Force
Dispersion forces are weak intermolecular forces caused by temporary dipoles.

Temporary Dipoles
Temporary dipoles are created when electrons, which are in constant movement around the nucleus, spontaneously come into close proximity. This uneven distribution of electrons can make one side of the atom more negatively charged than the other, thus creating a temporary dipole, even on a non-polar molecule. The more electrons there are in an atom, the further away the shells are from the nucleus; thus, the electrons can become lopsided more easily, and these forces are stronger and more frequent. These intermolecular forces are also sometimes called “induced dipole-induced dipole” or “momentary dipole” forces.

London Dispersion Forces
Although charges are usually distributed evenly between atoms in non-polar molecules, spontaneous dipoles can still occur. When this occurs, non-polar molecules form weak attractions with other non-polar molecules. These London dispersion forces are often found in the halogens (e.g., F2 and I2), the noble gases (e.g., Ne and Ar), and in other non-polar molecules, such as carbon dioxide and methane. London dispersion forces are part of the van der Waals forces, or weak intermolecular attractions.



 Reference 

Chemistry [master]. Lumen. (n.d.). Retrieved December 14, 2021, from https://courses.lumenlearning.com/trident-boundless-chemistry/chapter/intermolecular-forces/

Admin. (2020, October 21). Intermolecular Forces - definition, types, explanation & examples with videos. BYJUS. Retrieved December 14, 2021, from https://byjus.com/chemistry/different-types-of-intermolecular-forces/