1) When the two molecules are non polar there is a possibility of existing dispersion forces. This is due to the arrangement of electrons around the central atom of the molecule. Ex: n-pentane, Neopentane, n-octane etc When the two molecules are polar there exists dipole - dipole forces. Ex: It can be seen in acetone molecules When there is a hydrogen atom attached to the electronegative atom then there is a possibility of existing hydrogen bonding. Ex: H-F-------H-F 2) O,N,F are electronegative in nature which brings a large positive charge on the hydrogen atom. Moreover being smaller in size a large interaction forces results. 3) In diamond C is in sp3 hybridization and all the C\'s are at the same distance. It is in polymeric tetrahedral structure. Hence it is very hard in nature. Solution 1) When the two molecules are non polar there is a possibility of existing dispersion forces. This is due to the arrangement of electrons around the central atom of the molecule. Ex: n-pentane, Neopentane, n-octane etc When the two molecules are polar there exists dipole - dipole forces. Ex: It can be seen in acetone molecules When there is a hydrogen atom attached to the electronegative atom then there is a possibility of existing hydrogen bonding. Ex: H-F-------H-F 2) O,N,F are electronegative in nature which brings a large positive charge on the hydrogen atom. Moreover being smaller in size a large interaction forces results. 3) In diamond C is in sp3 hybridization and all the C\'s are at the same distance. It is in polymeric tetrahedral structure. Hence it is very hard in nature..