Intermolecular forces are responsible for most of the physical and chemical properties of matter. Although the exact nature of these forces are very complicated, there are no conceptual difficulties.
The basic building block in molecular compounds is the molecule, which is formed by relatively strong covalent bonds. Molecules are then held together in the solid and liquid states by weaker forces that have an effect on the physical properties of the material. Most of the theory as we know it today was developed during very early days of quantum mechanics. With the use of new types of quantum mechanical techniques and the availability of high-speed computing machines, it is now becoming possible to make accurate calculations of intermolecular forces.
The intermolecular forces if defined as
"The forces acting between the atoms due to electrostatic interaction between the changes of the atoms are called inter atomic forces".
During interaction between two atoms the following electrostatic forces will be active.
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Collectively, forces between molecules are called van der walls forces and these include the attractive and repulsive forces between
Intermolecular forces involve molecules that are in which polarity can be induced. Furthermore, several types of intermolecular forces can be at work in a single type of molecule. Each individual induced dipole/induced dipole force is usually quite small, the sum of these forces over the entire structure of a molecule can actually be quite even in polar molecules.
For example, the polar sulfur-dioxide molecule attracted one another in the following manner. Molecules of SO2 are polar with the partially negative region of one molecule being attracted to the partially positive region of an adjacent molecule.
The coulombic forces between molecules and atoms namely intermolecular forces, were described and defined by Van der waals in 1873. He invoked these intermolecular forces to explain why the behavior of real gases deviated from the ideal state. These are weal forces between atoms and molecules that do not involve the transfer or sharing of electrons, and therefore do not result in bonding. They are collectively known as Van der waals forces and can be divided into three categories.
Each of these interactions is weaker than interactions between fully charged ions, but they play an important role in determining the physical properties of molecules and their interactions with one another. In most cases a combination of forces acts together to contribute to the overall properties of the system so it is helpful to examine each of them to understand their relative importance and the factors that affect their strength.
In the case of molecules with no polar bonds the only attractive forces possible between the molecules are induced dipole-induced dipole interactions (London dispersion forces). For molecules with polar bonds, all three of the Van der waals forces contribute to the overall attractive forces: Induced dipole- induced dipole, Dipole-induced dipole and Dipole-dipole.
First, decide each substance is polar or non-polar. Second determine the types of intermolecular forces that could exist between the different pairs. Finally, use the 'like dissolves like' guidelines to decide whether iodine will dissolve in water or CCl4 and whether CCl4 will dissolve in water.
One component of van der waals forces is dipole-dipole attraction. The polar covalent bonding that the unsymmetrical distribution of electronic charges leads to positive and negative charges in the molecules, which are referred to as dipoles. In polar molecular substances, the dipoles line up so that the positive pole of one molecule attracts the negative pole of another. This is much like the lineup of small bar magnets. The force of attraction between polar molecules is called dipole-dipole attraction. These attractive forces are less than the full charges carried by ions in ionic crystals.
Another component of van der waals forces is called London force. Found in both polar and non polar molecules, it can be attributed to the fact that an atom that usually is non polar sometimes becomes polar because the constant motion of its electron may cause uneven charge distribution at any one instant.
London forces are about one-tenth the force of most dipole interaction and are the weakest of all the electrical forces that act between atoms or molecules. These forces help to explain why non polar substances such as noble gases and the halogens condense into liquids and then freeze into solids when the temperature is lowered sufficiently.
In general they also explain why liquids composed of discrete molecules with no permanent dipole attraction have low boiling points relative to their molecular masses. Because of the low intermolecular forces the melting points are low and evaporation takes place so easily that it may occur at room temperature. The following figure shows that the London forces are attractive forces.
A proton or hydrogen nucleus has a high concentration of positive charge. When a hydrogen atom id bonded to a highly electronegative atom, its positive charge will have an attraction for neighboring electron pairs. This special kind of dipole-dipole attraction is called a hydrogen bond. The more strongly polar the molecule is the more effective the hydrogen bonding is in binding the molecules into larger unit. As a result the boiling points of such molecules are higher than those of similar non polar molecules.
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