Question

Name the intermolecular force between
(i) He atoms in liquid He
​(ii) Two HF molecules

Answer 1

The correct answers have been given below - 

Ans (i) - The answer is - Vander Waal's forces. The intermolecular attractions in between two Helium atoms is very weak. The molecules will stick to each other to form a liquid only when the temperature is lower than 4 K or at -269°C. 

Ans (ii) - The answer is - Hydrogen bonds and Dipole-Dipole forces. HF is a polar molecule. The bonds that are formed in between the electronegative Fluorine atom and the Hydrogen atom is due to their dipole-dipole interaction and so the Hydrogen bonds are also a kind of dispersion forces.

Answer 2

i. He atoms in liquid He

The intermolecular forces between them are known as Van der Waals forces. These forces are much weaker than other types of bonding. They are a result of electrostatic attraction between temporary dipoles and induced dipoles caused by the movement of electrons in atoms and molecules.

ii. HF molecules

The intermolecular forces between them are known as Hydrogen bonds. They are permanent dipole - permanent dipole interactions. A hydrogen bond is stronger than other forms of permanent dipole - permanent dipole interactions. But it is weaker than a covalent bond. 

Bonds which have a hydrogen atom bonded to an atom of a strongly electronegative element like fluorine, nitrogen or oxygen are highly polar. Hydrogen bonds are electrostatic forces of attraction between molecules that contain these highly polar bonds.