Question

Assertion :(A) : Noble gases have very low boiling points. Reason: (R) : All noble gases have general electronic configuration of ${ns}^2{np}^6$ (except $He$)

• A. Both (A) and (R) are true but (R) is correct explanation of (A)
• B. (A) is false but (R) is true
• C. (A) is true but (R) is false
• D. Both (A) and (R) are true but (R) is not the correct explanation of (A)

Answer 1

The correct option is D Both (A) and (R) are true but (R) is not the correct explanation of (A)

The melting and boiling points of noble gases are very low in comparison to those of other substances of comparable atomic and molecular masses. This indicates that only weak van der Waals forces or weak London dispersion forces are present between the atoms of the noble gases in the liquid or the solid state.
The van der Waals force increases with the increase in the size of the atom, and therefore, in general, the boiling and melting points increase from He to Rn. Helium boils at −${269}^{\circ }C$. Argon has larger mass than helium and have larger dispersion forces. Because of larger size the outer electrons are less tightly held in the larger atoms so that instantaneous dipoles are more easily induced resulting in greater interaction between argon atoms. Therefore, its boiling point $(-{186}^{\circ }C)$ is more than that of He. Similarly, because of increased dispersion forces, the boiling and melting points of monoatomic noble gases increase from helium to radon.