Repulsive Forces VS Attractive Forces

In order for molecules to exist in aggregates in gases, liquids, or solids, there must be forces that attract the molecules together. This is a key concept in understanding the stabilization of emulsions, the compression of powders into capsules, and the attraction of the drug to the substrate in the body.

When the repulsive and attractive forces are equal the potential net energy is at a minimum and the system is stable.

Repulsive Forces

The force is repulsive when the molecules are brought close enough together that the outer charge clouds of the molecules touch, and this causes the molecules to repel each other. The repulsive forces are necessary so that the molecules do not destroy each other.

Attractive Forces

The forces that bring molecules together are called forces of attraction. These forces include cohesion, the attraction of like molecules, and adhesion, the attraction of unlike molecules. Attractive forces are divided into two groups: strong forces and weak forces. The weak forces of attraction are: Van der Waals forces, Ion-dipole forces, and Hydrogen bonds. The strong forces include the Ionic and Covalent bonds.

There are three different types of Van der Waals forces. They are Keesom, Debeye, and London. The Keesom forces are the attractive forces between permanent dipole molecules. The Debeye forces are the attractive forces between a permanent dipole molecule and an induced dipole molecule. An induced dipole molecule is a nonpolar molecule which was made (induced) to become a dipole molecule by the attractive force of another dipole molecule. London forces are attractive forces between two nonpolar molecules that when they come close together they induce each other to become dipole molecules. They are also called Induced dipole-Induced dipole forces.

Ion-dipole forces are attractions between ions and permanent dipoles. The attraction occurs because ions have a stronger charge than dipoles, so a partially charged end of a dipole will attract to an ion. this is important in the use of diuretics. Diuretics increase the volume of urine and remove excess electrolytes and fluid.

Ion-Induced dipole forces are attractions between ions and induced dipoles. For example, a potassium ion can induce a dipole in a diatomic iodine molecule. This is important in the solubility of iodine in water.

I₂ + K⁺I ^- -- >K⁺I₃^-

Hydrogen bonds are also weak forces of attraction. It is an attraction between a Hydrogen atom and a strong electronegative atom such as O, F, or N. These bonds are important in biological and chemical systems. For example, hydrogen bonds account for many of the unusual propertied of water such as high boiling point and high dielectric constant. Intermolecular and intramolecular hydrogen bonds can occur. Hydrogen bonds are relatively weak a in few numbers but are very strong in large numbers.

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