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

Short answer

The increase in boiling points of noble gases from He to Xe is due to the increase in polarizability (c).

Step-by-step solution

Understanding Boiling Point Trends

The boiling point of a substance is influenced by the attractive forces between its particles. In the case of noble gases, these are primarily van der Waals forces, which are related to the size and mass of the atoms.

Atomic Volume and Van der Waals Forces

Larger atoms have a greater volume, which increases the van der Waals forces because the electrons are more spread out and can be more easily polarized. Therefore, an increase in atomic volume can lead to an increase in boiling point.

Analyzing Electron Affinity

Noble gases have full valence shells, meaning their electron affinity is low because they typically do not gain electrons easily. Electron affinity does not directly influence boiling points in noble gases.

Assessing Polarizability

Polarizability refers to the ability of an electron cloud to be distorted, which increases with larger atomic size. More polarizable atoms have stronger van der Waals forces, leading to higher boiling points.

Evaluating Ionization Energy

A decrease in ionization energy means it's easier to remove an electron, which is not directly related to boiling points. For noble gases, ionization energy doesn't significantly affect the boiling point trends.

Conclusion from Analysis

Given these considerations, the increase in boiling points from He to Xe is primarily due to the increase in polarizability of the atoms, as larger atoms have more easily distorted electron clouds.

Key concepts

Van der Waals Forces

Noble gases, from helium to xenon, exhibit increasing boiling points, primarily due to the effect of Van der Waals forces. These forces are weak intermolecular forces that occur due to temporary dipoles created by the movement of electrons. In noble gases, Van der Waals forces are significant because other interaction forces, like ionic and covalent bonds, are absent.
As we move down the group in the periodic table, the size of noble gas atoms increases. This increased size contributes to stronger Van der Waals forces. Although these forces are weak compared to other intermolecular forces, they become more noticeable as the atomic number and size increase. This is because larger atoms provide more area over which these forces can act.
Hence, the greater atomic volume and increase in the number of electrons contribute to stronger Van der Waals forces, resulting in higher boiling points for heavier noble gases.

Atomic Size

The atomic size of ions significantly affects the boiling points of noble gases, and this is related to their increasing atomic volume as we move down the group. Larger atoms have more electron shells, which spread out the electrons over a larger volume. This spatial arrangement makes it easier for the electrons to induce temporary dipoles necessary for van der Waals interactions.
An increase in atomic size leads to an increased surface area for the electron cloud, allowing for better alignment with other atoms' electron clouds. As these temporary dipoles align, they increase the effectiveness of the van der Waals forces.

• Helium, being the smallest, has a very low boiling point due to minimal van der Waals forces.
• Xenon, larger and heavier, has a higher boiling point because of more substantial van der Waals interactions.

This suggests that the higher boiling points of noble gases as you progress down the group are most profoundly influenced by increased atomic size.

Polarizability

Polarizability is a crucial factor in understanding the boiling points of noble gases. It refers to how easily the electron cloud of an atom can be distorted. Larger atoms tend to be more polarizable because their electrons are farther from the nucleus and thus experience weaker effective nuclear charge.
The ability to distort the electron cloud increases as the atomic size increases. Consequently, larger atoms such as xenon have higher polarizability compared to smaller atoms like helium. Increased polarizability leads to stronger induced dipole-induced dipole interactions, which are a type of van der Waals forces.
Overall, this enhanced polarizability in larger noble gas atoms explains the stronger intermolecular attractions and, therefore, higher boiling points. Thus, as we move from helium to xenon, polarizability plays a critical role in the increasing trend in boiling points.