The term intermolecular force is used in Chemistry. The molecular attraction between the molecules of an atom due to the electron sharing is known as intramolecular force. However, the intramolecular force holds only for solid. Intermolecular force is the force that holds the molecules together and this hold true both for solid and liquid. These intermolecular forces can break easily because they are weak compared to the covalent bonds. An example for this would be, in order to break the intramolecular force for 1 mol of water it involves 927 kJ but, for the same experiment, only 41 kJ is required to break the intermolecular force for 1 mol of water and it changes to water vapour at 100°C. In very simple and easy words, the force that is responsible to hold the atoms together in a molecule is known as intramolecular force and the force that is present within the molecules is termed as intermolecular force.

Intermolecular forces are important because, they help to determine the melting points for a solid and in case of a liquid the boiling point. Liquids starts to boil, when the thermal energy within the molecules are enough to overcome the intermolecular attraction that holds the molecules together and thus, forming vapours which we can perceive within the liquid. In the same way the intermolecular force keeps the solid in place and this bond is broken when the thermal energy in high within the molecules and thus, solid melts. The nature of the intermolecular force is electrostatic, this implies that the force arises from the intercommunication between the positive and negative charges of the particular specie. These interactions are mostly important for liquid and solids because the intermolecular force helps to keep the molecules close together.

There are mainly three different types of intermolecular forces. One is the dipole-dipole interaction, the second one is the hydrogen bonding and the third one is known as London dispersion forces.

London dispersion forces

The force that is responsible for holding the molecules of the liquid, solid or for a solution phase together, is commonly known as London dispersion force and is generally quiet weak. These forces are found in the non-polar molecule and atoms. It was introduced by a German physicist, Fritz London, in the year 1930, and thus the name London dispersion forces. He proposed that within an atom and in non-polar molecules the temporary fluctuations during the electron distribution can result in the short lived immediate dipole moment, and result in attractive forces which are commonly known as London dispersion forces.

Considering a pair neighbouring He atoms as an example. The two electrons on an average are uniformly distributed around the nucleus. This happens due to the constant motion. The distribution in their one atom is likely to be asymmetrical for a given instant and thus results in an instantaneous dipole moment. The example given below explains the process.

Hydrogen bonding

In the periodic table the hydrogen is a special element. It is considered special because the hydrogen can act like a proton but can also form a special type of intermolecular bond or interaction which is known as hydrogen bond. In the periodic table the elements present are Nitrogen, Oxygen and Fluorine, hydrogen bond can be formed using these elements only. This bond happened because the partial negative end in oxygen, nitrogen, and fluorine are attracted to the partially positive end in hydrogen. The attraction force between the molecules is relatively strong and a considerable amount if energy is needed in order to break the hydrogen bonds. The hydrogen bonds explains the high boiling and melting points for the compounds like water (H₂​O), hydrogen fluoride (HF). The example below shows the hydrogen bond for H₂O.

In biology these hydrogen bonds play an essential role. In case of DNA and RNA these bonds are responsible to hold the nucleotide basses together.

Dipole-dipole bond

In case of a polar molecule, an example could be formaldehyde and the force that keeps these molecules together is known as dipole-dipole attraction. The polar molecules exhibit the dipole-dipole attraction. A dipole is generated when two atoms bond together. The example below show the dipole-dipole attraction of formaldehyde.

The example provided above show the different ways a dipole can exist. The (a) and (b) figure show the attraction force. In this case, molecules that have net dipole moments, incline to align themselves in a way that the positive end of one dipole is near the negative end of another and the opposite happened in case of the repulsive force.