Intermolecular forces| van der waal forces|

First of all we simply discuss that there are two major types of forces in chemical substances. 

1. Intramolecular forces
2. Intermolecular forces

Note: Here we will discuss about the intermolecular forces only.

Intermolecular forces:

As the word "inter" is clearly depicting the meaning of term "intermolecular forces".  Those forces which exist among molecules. These are weak forces and controlling the physical properties of substances. There are following types of intermolecular forces. and intermolecular forces are also known as van der waal forces.

Types:

1. Ion-dipole forces
2. Dipole-dipole forces
3. Hydrogen Bonding
4. London dispersion forces /Instentaneous dipole-induced dipole forces
   

 1.Ion-dipole Forces.

Ion-dipole Forces originate when an ion come across a polar substance.

For example when sodium chloride ( ionic compound) is dissolved in water( polar molecule) , ion -dipole forces occurs between sodium ion to water  and chloride ions to water. Water encircles the sodium ion by placing its negative end towards sodium ion(positive ion).Similarly water encircles the chloride ion( negative ion) by keeping its positive pole towards chloride ion( Cl-).

Note:

Ion- dipole forces cause the soulibility of ionic compounds into water . This solubility process is known as dissolution. Keep in mind if water is surrounding the ions ( ionic compound) this dissolution is known as "Hydration" and if any other polar substance os other than water this dissolution will be known as "solvation".

2. Dipole-dipole Forces:

Dipole-dipole forces exist among the polar molecules. Polar molecules are those molecules have permanent poles -positive and negative poles- due to electronegative difference between bonded atoms. hydrogen chloride (HCl),  hydrogen sulphide (H2S), etc

"When positive pole of a molecule attracts the negative end of other molecule a force is established known as "Dipole-dipole forces".

Dipole-dipole forces among hydrogen sulphide H2S molecules

3. Hydrogen Bonding:

 A special type of Dipole -dipole force emerge when specifically partial positive charged H atom attched with highly electronegative atoms and another nearby electronegative  atoms like Flourine(F), Oxygen(O) and Nitrogen (N) of other molecule. This special type of dipole- dipole force is known as Hydrogen bonding. Remember hydrogen bonding is associated with just three highly electronegative atoms i.e. flourine (F), Oxygen (O), and Nitrogen(N). common molecules have hydrogen bonding are Water H2O, Hydrogen Flouride (HF), and ammonia (NH3).

Hydrogen Bonding among Hydrogen Flouride(HF) Molecules

Hydrogen Bonding among Water H2O Molecules

Hydrogen bonding is the strongest among the intermolecular forces but remember it is weaker than intramolecular forces i.e covalent bonding and ionic bonding.

4. London dispersion forces | instantaneous dipole-induced dipole forces|

 In case of non polar molecules a special kind of intermolecular force originates among molecules called instantaneous dipole-induced dipole force or London dispersion forces. Non polar molecules have symmetrical electrons distribution and by chance develop a pole when constantly moving electrons gather at one end and create negative pole at one end than the other. Then molecules with unsymmetrical electron distribution have temporary poles called polarized molecules.This polarized molecule have ability to induce a dipole in another near molecule. In such way polarized molecules induced poles in other molecules so called instantaneous dipole-induced dipole forces as well as London forces.

Pictorial description of the London forces

London forces are weakest forces among intermolecular forces. Its strength depends on the polarizibility of molecules. Bigger molecules develop more stronger London forces.

Polarizibility:

Polarizibility is the quantitative measurement of molecules how much a molecule can be distorted. Larger molecules have  more ability to distort hence have more polarizibility.

These forces occur in halogens ( F2,Cl2,Br2 and I2), nobel gases (He,Ne,Ar,Kr) Carbondioxide (CO2) ,Methane (CH4) and other non-polar molecules.