Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The ease of liquefaction of noble gases increase in the order (a) \(\mathrm{Ar}>\mathrm{Kr}>\mathrm{Xe}>\mathrm{Ne}>\mathrm{He}\) (b) \(\mathrm{Xe}>\mathrm{Kr}>\mathrm{Ne}>\mathrm{Ar}>\mathrm{He}\) (c) \(\mathrm{Kr}>\mathrm{Xe}>\mathrm{He}>\mathrm{Ne}>\mathrm{Ar}\) (d) \(\mathrm{He}>\mathrm{Ne}>\mathrm{Ar}>\mathrm{Kr}>\mathrm{Xe}\)

Step-by-Step Solution

Answer

The ease of liquefaction increases in the order: (d) He < Ne < Ar < Kr < Xe.

Step 1: Understanding Liquefaction of Noble Gases

Liquefaction refers to the process of converting a gas into a liquid state. The ease of liquefaction for noble gases depends on their intermolecular forces, primarily van der Waals forces. Noble gases with larger atomic radii have stronger van der Waals forces, making them easier to liquefy.

Step 2: Analyze Atomic Size and van der Waals Forces

Noble gases are arranged in the periodic table as He, Ne, Ar, Kr, and Xe with increasing atomic size from helium to xenon. Helium has the smallest atomic radius and weakest van der Waals forces, while xenon has the largest atomic radius and strongest van der Waals forces. Therefore, the ease of liquefaction generally increases from helium to xenon.

Step 3: Identify the Correct Order of Noble Gases

Considering the atomic size and the associated van der Waals forces, the noble gases should be arranged in order of their increasing ease of liquefaction as: He < Ne < Ar < Kr < Xe. This order is consistent with the option provided in choice (d).

Key Concepts

Intermolecular Forces

Intermolecular forces play a crucial role in determining the physical states of materials, such as gases and liquids. For noble gases, these forces are relatively weak compared to other substances because noble gases exist in a monoatomic form, meaning they are comprised of single atoms rather than molecules. However, even these single atoms experience a type of intermolecular force known as van der Waals forces. The strength of these forces significantly influences the ease with which noble gases can transition from a gaseous to a liquid state. In the context of the liquefaction of noble gases, stronger intermolecular forces mean that the gas atoms interact more robustly with one another. This interaction facilitates the atoms sticking together more easily, thereby enhancing the transition to a liquid state. For noble gases, these forces are weak but become critical in understanding their physical behavior at low temperatures.

Atomic Size

Atomic size, or atomic radius, is a key factor that affects the behavior of noble gases and influences their liquefaction. Atomic size refers to the distance from the center of an atom's nucleus to its outermost electron shell. Among the noble gases, atomic sizes increase from helium, which is the smallest, to xenon, the largest. Larger atomic size leads to stronger van der Waals forces because more electrons are present, and these electrons can polarize more easily upon interaction with nearby atoms. As a result, noble gases with larger atomic radii, like xenon, have stronger interactions and are more readily liquefied than their smaller counterparts, such as helium or neon. When considering atomic size in the context of liquefaction, it is crucial to note that the trend in atomic size directly correlates with the ease of liquefaction – larger atoms are easier to liquefy due to the enhancement of intermolecular forces.

Van der Waals Forces

Van der Waals forces encompass the weakest type of intermolecular forces, vital in influencing the physical states of noble gases. These forces arise due to the temporary and small polarization of atoms, which results in interactions even in gases like helium and xenon that are often considered chemically inert. The significance of van der Waals forces becomes evident when observing the liquefaction process of noble gases. Since noble gases do not form conventional chemical bonds, van der Waals forces become the primary interaction facilitating the transition from gas to liquid. As the atomic size increases, the polarizability, or the ability of the electron cloud to become distorted, also increases. This results in stronger van der Waals forces. These forces are largely responsible for the order of liquefaction ease observed in noble gases: helium < neon < argon < krypton < xenon. The increase in atomic size is directly linked to the increased van der Waals interactions, making it crucial in understanding why larger noble gases liquefy more readily.