Intermolecular forces - definition and types

What are intermolecular forces?

Intermolecular forces are defined as the set of attractive and repulsive forces that occur between the molecules as a result of the polarity of the molecules.

When two or more atoms are joined by chemical bonds they form a molecule, electrons travel up to the new molecule and are concentrated in the most electronegativity atom area, the electronegativity is defined as property that have the atoms or molecules to attract electrons. The concentration of electrons in a defined area of the molecule creates a negative charge, while the absence of electrons creates a positive charge.

Dipoles are molecules that have negative and positive charged areas due to the electronegativity and concentration of electrons in molecules.

We can assimilate the action of a dipole with a magnet with its positive and negative charge, so that if we bring another magnet will attract the positive to negative and viceversa, resulting a bond.

intermolecular forces

Intermolecular forces acting between the molecules are classified as:

Within the 4 groups described above, the most important forces are the top 3, also known as Van der Waals forces.

Permanent dipoles

This type of bonding occurs when two molecules have positive and negative charges, they are polar molecules that have polarity, electrostatically attracting and forming the bond.

Induced dipoles

This type of bonding occurs when a non polar molecule redistributes the concentration of electrons (it has the ability to polarize) when it approach of a polar molecule, so that it creates a bond between two molecules.

In this case the polar molecule induces the creation of the non-polar molecule in a polar molecule.

Dispersed dipole

This latter case the binding occurs between polar molecules but can be polarized, and when the latter occurs attract each other creating a molecular bond.

The bond that is created with this type of dipole it has a very weak intensity and short lived.

The binding energies generated by the intermolecular forces are smaller than the energies generated in the chemical bonds, but exist in greater numbers compare with the number of chemical bonds, so that globally involves a very important role.

Van der Waals ------ 0.1 to 10 kJ / mol

Covalent bond ------ 250-400 kJ / mol.

The following table is a comparison between the properties of the intermolecular forces and the chemical bonds:

Intermolecular Forces:

Chemical bonds:

  • Intermolecular forces depend on the temperature, an increase of the temperature produces a decrease of the intermolecular forces.

  • They are weaker than chemical bonds, order of 100 times lower

  • The bond distance is at the level of microns

  • Unions are not directed.

  • Chemical bonds do not depend so much about the temperature.

  • They are stronger than intermolecular forces

  • The bond distance is very small, in terms of Angstroms

  • Unions are directed

With all this, inside adhesive material is a polymer as we find chemical bonds between atoms in molecules and intermolecular forces between the molecules themselves of the polymers. All these bonds, joints and forces are responsible for the adhesion and cohesion properties of glues, adhesives and sealants.

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