Intermolecular ForcesβThe forces between the molecules of a substance are known as intermolecular forces. Such forces exist in all states of matter and are responsible for many structural features and physical properties of matter. The intramolecular forces on the other hand exist within each molecule (or polyatomic ion) and influence the chemical properties of the substance.
Intermolecular Forces are Weak Forces
It usually requires, for example, much less energy to evaporate a liquid than to break the bond in the molecules of the liquid. The boiling points of substances often reflect the strength of intermolecular forces operating among the molecules. At the boiling point, enough energy must be supplied to overcome the attractive forces among molecules before they can enter into vapour phase. Similarly, the melting point of substances increases with increase in the strength of the intermolecular forces.
Intermolecular forces arise due to the following types of reactions: dipole-dipole, ion-dipole, ion-inducted dipole, dipole-induced dipole, dispersion of forces and Hydrogen bonding. The term 'Van der Waals forces' refers to dipole-dipole, dipole-induced dipole and dispersion forces. Depending on the phase of the substance, the nature of chemical bonds and the types of element present, more than one type of interaction may contribute to the total attraction between molecules.
Dipole-dipole interactions depend upon the distance and orientation of the two dipoles. A favourable orientation of the two dipoles results in attractive dipole-dipole interactions. These exist between molecules having permanent dipoles.
Ion-dipole interactions attract an ion (either a cation or an anion) and a polar molecule to each other. The strength of this interaction depends on the charge and size of the ion and on the magnitude of the dipole moment and size of the polar molecule. The charge density on cations is higher than on anions, therefore a cation interacts more strongly with dipoles than does an anion having same charge but bigger size. Hydration of different ions is an example of ion-dipole interaction. In an aqueous NaCl solution, the Na+ and Cl- ions are surrounded by water molecules, which have a large dipole moment. When an ionic compound like NaCl dissolves, the water molecules act as a dielectric to keep the ions apart. On the other hand, carbon tetrachloride (CCl4) a nonpolar molecule lacks the ability to participate in ion-dipole interaction. Therefore, carbon tetrachloride is a poor solvent for ionic compounds, as are most non polar liquids.
Ion-Induced Dipole Interactions and Dipole-Induced Dipole Interactions
The attractive interaction between an ion and the induced dipole is called ion-induced dipole interaction, and the attractive interaction between a polar molecule and the induced dipole is called dipole-induced dipole interaction. The likelihood of a dipole moment being induced depends not only on the charge on the ion or the strength of the dipole but also on the polarizability of the atom or molecule i.e., the ease with which the electron distribution in the atom (or molecule) can be distorted.
Dispersion forces are the attractive forces between nonpolar substances such as O2, N2 or monotomic gases such as He, Ar etc. These arise as a result of instantaneous dipoles created in these nonpolar molecules or atoms. Let us understand the origin of these forces, say in helium gas. In a helium atom the electrons are moving at some distance from the nucleus. At any instant it is likely that the atom has a dipole moment created by the specific positions of electrons. This dipole moment is called an 'instantaneous dipole' because it lasts for just a fleeting moment. In the next instant the electrons are in different locations and the atoms have a new instantaneous dipole, and so on. Averaged over a time (that is, the time it takes to make a dipole moment measurement), however, the atom has no dipole moment because the instantaneous dipoles all cancel one another. In a collection of He atoms, an instantaneous dipole of one He atom can induce a dipole in each of its nearest neighbours. At the next moment, a different instantaneous dipole can create temporary dipoles in the surrounding He atoms. This kind of interaction produces attractive dispersion forces between He atoms. At very low temperatures, dispersion of forces is strong enough to hold He atoms together, causing the gas to condense. The attraction between non polar molecules can be explained similarly.