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

Question

The increase in boiling points of noble gases from He to 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

(c) increase in polarizability

Step 1: Understanding the Question

The problem asks why the boiling points of noble gases increase as we move from Helium (He) to Xenon (Xe) in the periodic table. We need to identify the factor responsible for this trend among the given options.

Step 2: Analyzing Noble Gas Properties

Noble gases are non-metals with complete outer electron shells. They experience very weak intermolecular forces known as London dispersion forces. These forces depend on the electron cloud's polarizability, which can be affected by the atomic size and the number of electrons.

Step 3: Evaluating Polarizability

Polarizability refers to how easily an electron cloud can be distorted. Larger atoms with more electrons tend to have more easily distorted electron clouds, increasing their polarizability, which results in stronger London dispersion forces and higher boiling points.

Step 4: Correlating Atomic Size and Polarizability

As we move from Helium to Xenon, atomic size increases due to additional electron shells, leading to greater polarizability. This increase in polarizability strengthens the dispersion forces, thus raising the boiling points of the gases.

Step 5: Conclusion

Given the relationship between atomic size, polarizability, and boiling points, the increase in boiling points from Helium to Xenon is attributed to the increase in polarizability.

Key Concepts

Boiling Points

Noble gases are known for their distinct characteristic of having complete electron shells. This feature plays a pivotal role in their boiling points. The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, leading to the transition from a liquid to a gaseous state.

For noble gases like Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), and Xenon (Xe), the boiling points increase as we move down the periodic table from lighter to heavier gases. This trend seems peculiar at first since noble gases are monoatomic and unreactive, but it is linked to their atomic properties.

• Heavier noble gases have more electrons.
• They show an increase in atomic size as additional electron shells are added.
• This results in the larger electron clouds having greater polarizability, thereby strengthening the intermolecular forces.

Thus, the reason behind the increase in boiling points from He to Xe lies primarily in the enhancement of these intermolecular forces, specifically due to their polarizability.

Polarizability

Polarizability is a key concept in understanding how atoms and molecules interact with each other. It refers to the ability of an atom's electron cloud to be distorted by external forces. In practical terms, it determines how easily the shape of the electron cloud can be changed, which in turn influences intermolecular interactions.

Larger atoms, such as those found in heavier noble gases, have more electrons and additional electron shells. This results in a greater volume for the electron cloud to occupy, making it more flexible and easier to distort.

• As atomic size increases, so does polarizability.
• Greater polarizability enhances the strength of London dispersion forces among the atoms.

Consequently, polarizability is directly tied to various physical properties of atoms, including boiling and melting points. The relationship between atomic size, polarizability, and boiling points is directly visible in the noble gases, where the increase in polarizability from Helium to Xenon is matched by a rise in boiling points.

London Dispersion Forces

London dispersion forces are a type of weak intermolecular force that is present in all atoms and molecules. They are particularly significant in noble gases due to their non-reactive nature and monoatomic form. These forces arise from temporary shifts in electron density that create instantaneous dipoles in atoms.

The strength of London dispersion forces increases with polarizability; therefore, larger atoms with more electrons and a more extensive distribution of electron cloud form stronger dispersion forces. As a result, as we move from Helium to Xenon:

• The atomic size increases.
• The electron cloud becomes more polarizable.
• Stronger London dispersion forces are formed.

This results in an increase in the boiling points of these gases. Thus, the progression of boiling points among noble gases is a direct reflection of the increasing effectiveness of London dispersion forces with larger atoms.