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

Short answer

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

Step-by-step solution

Analyze the behavior of noble gases

Noble gases are elements in Group 18 of the periodic table. They have full outer electron shells, making them very unreactive. However, as we move down the group from Helium (He) to Xenon (Xe), there is a noticeable increase in boiling points. This trend needs to be explained by considering what changes in the atomic structure or properties might affect the boiling point.

Consider atomic volume and boiling points

The atomic volume generally increases as we move down a group in the periodic table. Larger atoms tend to have weaker van der Waals forces due to increased distance between nuclei and electrons in adjacent atoms. Since boiling point is related to the strength of these intermolecular forces, option (a) alone might not be a direct reason for the increase in boiling points.

Evaluate electron affinity

Electron affinity refers to the tendency of an atom to accept an electron. Noble gases have very low electron affinities because their outer shells are full, so changes in electron affinity are not significant in describing the trend in boiling points. Thus, option (b) is unlikely to be the cause.

Analyze polarizability

Polarizability is the ability of an atom's electron cloud to be distorted. As atomic size increases, polarizability increases, leading to stronger induced dipole interactions (van der Waals forces). This increase in cohesive forces requires more energy (higher temperature) to overcome, thereby increasing the boiling points. So, polarizability is a key factor here, making option (c) a strong candidate.

Consider ionization energy

Ionization energy typically decreases as you move down a group because outer electrons are farther from the nucleus. A decrease in ionization energy corresponds to easier removal of electrons, but this does not directly relate to changes in boiling points due to noble gases' stability; therefore, option (d) does not justify the increase in boiling points.

Conclusion: Finalize the answer

Based on our analysis, the increase in boiling points of noble gases from He to Xe is primarily due to the increase in polarizability with larger atomic sizes, which enhances van der Waals forces.

Key concepts

Atomic Volume

Atomic volume refers to the space occupied by an atom. It generally increases as we move down a group in the periodic table. For noble gases, this means each subsequent element from Helium (He) to Xenon (Xe) is larger in size. As the atomic volume increases, so does the physical size of the atoms. This can affect their other properties as well. When we consider boiling points, larger atomic volume can lead to weaker interactions when considering only simple distance logic, since more space exists between the centers of adjacent atoms. However, other factors like polarizability also play a critical role in these interactions. The increase in atomic volume alone does not have a direct and strong effect on the boiling points of noble gases but it sets the stage for increased polarizability. It's important to remember that while atomic volume is a factor to consider, it is the resulting increase in polarizability that has the most impact on the boiling points of noble gases.

Polarizability

Polarizability is a measure of how easily the electron cloud around an atom can be distorted by external forces, such as the presence of nearby charges. In noble gases, polarizability increases as the atomic size increases. This means that gases like Xenon (Xe), which are larger, have more polarizable electron clouds compared to smaller ones like Helium (He). As polarizability increases, so does the strength of induced dipole-dipole interactions, often referred to as van der Waals forces. This is because a more easily distorted electron cloud can form temporary dipoles more readily. The increased polarizability of larger noble gases leads to stronger intermolecular attractions. This means more energy is needed to separate the atoms into a gaseous state, hence a higher boiling point. Therefore, polarizability is a key factor in explaining the trend in boiling points of noble gases as we move down the group in the periodic table.

Van der Waals Forces

Van der Waals forces are weak attractive forces between molecules. They become significant in explaining properties like boiling points, especially in nonpolar substances such as noble gases. These forces include interactions arising from temporary dipoles induced in adjacent atoms or molecules. Because noble gases are nonpolar, they rely primarily on van der Waals forces for intermolecular interactions. As mentioned, polarizability plays an essential role in the strength of van der Waals forces. Larger atoms with bigger electron clouds are more easily polarizable, leading to stronger van der Waals forces. In noble gases, as we move from He to Xe, the increasing atomic size results in stronger van der Waals forces due to enhanced polarizability. This means more heat (or energy) is needed to overcome these interactions to move the atoms from a liquid to a gas phase, contributing to higher boiling points.