Question

Gradual addition of electronic shells in the noble gases causes a decrease in their
A. Ionisation energy
B. Atomic radius
C. Boiling point
D. Density

Answer 1

Hint: In a group, when we progress there is an addition of new subshells. Electrons get added to these shells. Atomic radius or atomic size increases down the group.

Complete Step by Step Solution:
We know that down a group, atomic radius increases. The valence electrons are added to new levels as the principal quantum number (n) increases. Consequently, the valence electrons are far away from the nucleus as ‘n’ increases.

Electron shielding or screening prevents these outer electrons from being drawn to the nucleus hence, they are lightly held. So, the atomic radius becomes large. Here in this question, we have to find out whether the gradual addition of electronic shells in the noble gases causes a decrease in their
A. Ionisation energy
As down the group, the electrons are loosely held, Ionisation energy decreases. This is because less energy is required to eliminate an electron from the outermost shell. So, the gradual addition of electronic shells in the noble gases causes a decrease in their Ionisation energy. So, A is correct.

B. Atomic radius
Atomic radius increases down the group. So, B is incorrect.

C. Boiling point
We know that noble gases have weak van der Waals forces. The strength of these forces increases as we move down the group due to an increase in no.of electrons. As the intermolecular forces increase, the boiling point increases. So, C is incorrect.

D. Density
We know that density is the ratio of mass to volume. As we progress down the group, atomic size grows. The volume of the atom rises as it is directly proportional to atomic size. But the rise in mass is more than the gain in volume hence, density increases. So, D is incorrect.
So, option A is correct.

Note: Neon is a constituent of neon lamps and signs. Giving an electric spark through a tube comprising neon at low pressure develops the red glow of neon. The change in the colour of the light is done by combining argon or mercury vapour with neon or by using glass tubes of a unique colour.