Matters are composed of discrete particles, which could be distinct atoms or molecules. The particles of matter attract each other, this property is known as cohesion and is the reason for the elasticity of the material. The attraction between particles varies in different matters. This attractive force is highest in solids, followed by liquid and gas.
Examples of Forces between Particles
Few examples can demonstrate the existence of the force of attraction between particles and the degree of attractive force between the particles of matter.
If you try to bend iron with your bare hand it will be impossible, however, you can easily break a piece of chalk or pencil. This is because the attraction between iron molecules is much greater than that of chalk pieces.
The elasticity of a rubber band can also be attributed to the cohesive property of the material. When you stretch a rubber band, the external force applied overcomes the attractive force between the molecules, as long as this pull is maintained the band remains stretched, but, once the external force is removed, the intermolecular force of attraction acts to put the rubber band back to its original shape. If the band is pulled beyond a certain limit (breaking point), it can cause permanent rupture and tear the rubber band.
If you move your hand through the water, you will experience resistance, but you can move your hand through air relatively easily. The force of attraction between water molecules is much more than that in air. The particles in liquids are somewhat more compressible than the particles in gases.
The surface tension of water is a property that arises due to the attractive force between the molecules of water. The tension is the result of an attractive force that acts on a water molecule on the surface. The water molecules at the surface experience a pull inward and tangentially on the surface. The tangential force on the molecule on the surface is the surface tension, it makes the surface behave like an elastic membrane.
Types of Molecular Interaction
But, why do particles of matter attract each other? There exists a weak force attraction between two molecules of a substance. This force of attraction is called the intermolecular force of attraction. It gives the substance its unique physical properties. The force of attraction between particles of matter can arise due to a number of interactions such as:
Dipole-induced dipole interaction
Ion-induced dipole interaction
The intermolecular forces due to Dipole-dipole interaction, dipole-induced dipole interaction and dispersion forces are often called Van der Waal forces. It arises due to electrical interaction between two molecules.
The electron cloud around a molecule continuously fluctuates causing temporary polarisation of charges. As the electron cloud shifts from one side to another, the charge difference results in the formation of dipoles. These temporary dipoles of one molecule interact with another molecule close by to give rise to attractive forces.
Polar molecules having permanent dipoles have dipole-dipole interaction. Uncharged molecules develop transient dipoles due to the spontaneous motion of electron clouds; when an instantaneous dipole gets closer to another molecule it induces a charge separation in that molecule and gives rise to an induced dipole. This is Dipole-induced dipole interaction. The poles of the dipoles fluctuate in synchrony to maintain the interaction. The induced dipole moment can influence other molecules, which can influence others, this way the induced dipole can disperse across a large network molecule in a system. This is known as dispersion forces.
Molecules composed of ions can similarly have ion-dipole interaction and ion-induced dipole interaction. An ion that has a distinct charge can interact with polar molecules to give rise to ion-dipole interaction, for example when sodium chloride dissolves in water, the sodium cation or chloride anion is surrounded by the polar water molecules, this is called hydration of ions and leads to the dissolution of sodium chloride crystals.
Ions can also induce permanent dipoles in a nonpolar molecule by perturbing its electron cloud. This results in ion-induced dipole interaction.
Hydrogen Bonding and Van der Waal Forces
When a partially positively charged hydrogen is attracted by another partially negatively charged atom it results in hydrogen bonding. An extensive hydrogen bonding exists in water that gives it its unique property.
The extent of van der Waal forces, hydrogen bonding and other interactions determine the physical property of a substance such as the boiling point and the melting point. Substances with higher van der Waal forces, and hydrogen bonding will have a higher boiling point. More heat will be required to break the interactions between the molecules or the hydrogen bonds and disrupt the order to bring about a change in state.
The magnitude of attraction between the particles of matter is a significant determinant of the chemistry of the substance.
Matters are composed of discrete particles like molecules or atoms
Particles of matter attract each other
The weak force of attraction exists between molecules of a substance
The intermolecular force of attraction determines the physical nature of a substance