Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

The increase in boiling points of noble gases from He to \(\mathrm{Xe}\) is due to the (a) increase in atomic volume (b) increase in electron affinity (c) increase in polarizability (d) decrease in ionization energy

Step-by-Step Solution

Answer

The increase in boiling points is due to (c) increase in polarizability.

Step 1: Understanding Noble Gases

Noble gases are a group of chemical elements with similar properties. They are in group 18 of the periodic table. The elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They have low boiling points because they exist as monoatomic gases with very weak interatomic forces, typically van der Waals forces.

Step 2: Properties Affecting Boiling Points

The boiling point of noble gases depends on the strength of intermolecular forces. As atomic size increases from helium to xenon, the atomic volume and the atomic mass of the gases increase, causing an increase in polarizability, which leads to stronger van der Waals forces or London dispersion forces.

Step 3: Analyzing Options

Let's analyze each option: (a) Increase in atomic volume leads to stronger dispersion forces. (b) Electron affinity increase does not significantly alter boiling point. (c) Increase in polarizability directly contributes to stronger van der Waals forces and higher boiling points. (d) Decrease in ionization energy generally affects reactivity but not significantly the boiling points.

Step 4: Identifying the Correct Concept

Among the options, (c) the increase in polarizability is directly related to the strength of van der Waals forces and consequently the boiling points. As atomic number and volume increase, the effect of induced dipoles becomes more pronounced, resulting in stronger intermolecular attractions.

Key Concepts

Noble Gases

Noble gases are fascinating elements found in Group 18 of the periodic table. This group consists of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These elements are known for being inert, which means they have little to no reactivity under normal conditions. Noble gases exist as single atoms, not forming bonds with other atoms easily, due to their complete electron shells. This complete outer shell gives them a very stable electronic configuration.

Since noble gases have very low boiling points, they are mostly found in gaseous form under standard conditions. Boiling points tend to increase as you move down the group from helium to radon. This shift is an outcome of increasing atomic size and mass, which directly impacts the intermolecular forces present among the atoms.

Boiling Points

The concept of boiling point is essential in understanding how substances transition from one phase to another. Specifically, it is the temperature at which the vapor pressure of a liquid equals the external pressure surrounding the liquid. For noble gases, boiling points increase from helium to xenon. This trend is linked to the increase in atomic size and mass as we move down the group.

The strength of intermolecular forces is a key factor influencing boiling points. In the case of noble gases, as each successive element in the group has more electrons, the overall polarizability of the atom increases, which in turn, raises the boiling point. Despite being generally weak in comparison to those of other atoms, the van der Waals interactions play a crucial role here. These forces are more significant in larger atoms, leading to higher boiling points.

• Boiling points rise due to stronger van der Waals forces.
• These forces are enhanced by the increased polarizability of the gas atoms.

Polarizability

Polarizability refers to the ease with which the electron cloud of an atom or molecule can be distorted by an external electric field. In noble gases, polarizability increases as you move down the group. Larger atoms have more electrons, which are held less tightly by the nucleus, allowing their electron clouds to become more easily distorted.

Increased polarizability enhances the strength of van der Waals forces. This is a crucial factor because even though noble gases have no permanent dipole moments, temporary dipoles can be induced due to electron cloud distortion. As a result, atoms can stick together more effectively under these induced forces, thereby increasing their boiling points.

Here are some points to consider:

• Larger atoms with more electrons are more polarizable.
• Greater polarizability leads to stronger induced dipoles.

Van der Waals Forces

Van der Waals forces are a type of intermolecular force that influences the interactions between noble gas atoms. These are weak forces compared to other types of chemical bonds, but they are significant in understanding the behavior of gases. In noble gases, these forces primarily result from London dispersion, a type of van der Waals force.

As atomic size and electron count increase down the group, the effects of van der Waals forces become more pronounced. A larger electron cloud can exhibit more momentary distortions, creating stronger temporary dipoles, which increase the attraction between atoms.

• Van der Waals forces are crucial for the existence of condensed phases like liquids and solids in noble gases.
• They contribute directly to the increase in boiling points as you move from helium to xenon in the periodic table.

Thus, understanding van der Waals forces helps explain why noble gases with larger atomic size tend to have higher boiling points.