Intermolecular Interactions Distance Dependence

Interactions between two or more molecules are called intermolecular interactions, while interactions between molecular atoms are called intramolecular interactions. Intermolecular interaction occurs in all types of molecules or ions in all states of matter. They range from strong long-range electrical attraction and repulsion between ions to relatively weak dispersion forces that have not yet been fully explained.

Without these interactions, condensed forms of matter (liquids and solids) exist only at extremely low temperatures. We will explore these different forces and interactions in the gas phase to understand why some materials vaporise at extremely low temperatures, while others persist in solid or liquid form at extremely high temperatures.

Intermolecular Forces Meaning

Intermolecular forces can be described as the attractive and repulsive forces that exist between interacting particles of atoms and molecules in a compound. Attractive intermolecular forces are called van der Waals forces and include dispersion or London forces, dipole-dipole forces, and dipole-induced dipole forces.

The intermolecular forces in the solid  are very strong. The constituent particles are tightly packed, making the solid incompressible and dense. In the case of liquids, intermolecular forces hold the particles together but do not hold them in a fixed position, allowing them to flow easily and have a certain shape. In the case of gases, the intermolecular forces are very weak, allowing the constituent particles to move freely.

Dispersion Forces or London Forces

The London Force or Dispersion Force refers to the attractive force that exists between two-time forces. It was first proposed by the German physicist Fritz London. These forces are always attractive. The interaction energy of this force to the sixth power of the distance between the two interacting forces is inversely proportional.

Consider two atoms and consider the electron charge distribution in an asymmetric atom. The charge cloud is denser on one side than the other. This results in the creation of direct dipoles on the atoms for a short period of time. This creates a dipole in the atom.

The temporary dipoles of both atoms attract each other. Similarly, temporary dipoles are created in molecules. These forces apply to short distances of about 500 µm. The magnitude of this force depends on the polarizability of the interacting particles.

Dipole-Dipole Forces

The dipole-dipole force is the force that exists between molecules with permanent dipoles, where each end of the dipole consists of a partial charge that is always less than the unit electron charge (1.6 × 10-19 C). This force is stronger than the London force but weaker than the ion-ion interaction due to the participation of partial charges.

The attractive force decreases as the distance between the dipoles increases. The distance between polar molecules and the interaction energy of this force is inversely proportional to each other.

Dipole induced dipole force

Dipole-induced dipole force is the force between polar molecules with permanent dipoles and molecules without permanent dipoles. Permanent dipoles of polar molecules induce dipoles on electrically neutral molecules by creating distortions in their electron clouds.

The interaction energy of this force to the sixth power of the distance between the two molecules is inversely proportional. The higher the polarisability, the greater the attraction.

Hydrogen Bonds

Some substances such as H2O, HF, and NH3 will form hydrogen bonds that affect the substance’s properties (mp, bp, solubility). Other compounds containing OH and NH2 groups can also form hydrogen bonds. The molecules of many organic compounds such as alcohols, acids, amines, and amino acids contain these groups, so hydrogen bonding plays an important role in biological sciences.

Covalent Bonds

Covalent bonds are actually intramolecular rather than intermolecular forces. It is mentioned here because some solids form due to covalent bonds. For example, in diamond, silicon, quartz, etc., all the atoms of the entire crystal are bound together by covalent bonds. 

Metal Bonds

The forces between atoms in metallic solids fall into different categories. Valence electrons are everywhere in metals. They are not limited to specific atoms or bonds. Instead, they flow freely in solids and provide good conductivity for thermal and electrical energy.

Conclusion

All intermolecular interactions are electrostatic, that is, they are caused by the attraction of opposite charges. The strength of these interactions increases with an increase in charge. Ions are the most charged, so interactions formed between oppositely charged ions are the strongest intermolecular interactions.

The second strongest is the interaction between molecules containing polar bonds. Polar bonds are covalent bonds in which electrons are shared unequally. Hence, atoms that are more electronegative have a partial negative charge, and atoms that are less electronegative have a partial positive charge.

This arrangement, in which positive and negative charges are separated between atoms, is called dipoles. The intermolecular forces are responsible to some extent for the properties exhibited by matter.