Question

Which of the following exhibits the weakest intermolecular forces?
A. $ {{\text{H}}_{\text{2}}}{\text{O}} $
B. $ {\text{N}}{{\text{H}}_{\text{3}}} $
C. $ {\text{He}} $
D. $ {\text{HCl}} $

Answer 1

Hint:The forces which mediate the interaction between molecules, including the forces of attraction or the repulsion which act between molecules and other types of particles such as ions. Mostly the elements which have complete octet in its elementary form exhibit the weakest intermolecular forces.

Complete answer:
Among the above molecules, the forces of interaction between the water and ammonia molecules are hydrogen bonding as well as van der Waals attraction.
The force of interactions between the hydrogen chloride molecules are dipole-dipole interactions while the interactions between the helium atoms is just weak van der Waals attraction.
Hence, the weakest intermolecular forces exist between the helium atoms as there are only van der Waals interactions.

Thus, the correct option is C.

Note:
There are different types of intermolecular forces of attraction depending on the type of molecules. These maybe listed as follows:
Hydrogen bonding – The force of attraction between the lone pair of an electronegative atom like nitrogen, oxygen, and fluorine and a hydrogen atom which is bonded to an electronegative atom. These types of interactions are moderately strong in nature.
Ionic Bonding – The force of attraction that exists anions and cations is called the ionic bond. This bond is very strong that keeps the anions and cations bonded together in a molecule.
Dipole-dipole and dipole-induced interactions – A polar covalent molecule that has atoms with difference in electronegativities is called a dipole. The interactions between dipoles of opposite charges are strong in nature.
Van-der Waals forces – These forces of attraction are very weak forces of interactions and are present in most of the molecules.