Why do the boiling points of the noble gases increase in the order He \(<\mathrm{Ne}<\mathrm{Ar}<\mathrm{Kr}<\mathrm{Xe}\) ?

Intermolecular forces (IMFs) are the attractions between molecules, which determine many physical properties of a substance, including boiling point, melting point, and volatility. Unlike intramolecular forces that bind atoms together to form molecules, intermolecular forces cause molecules to stick together.

In the context of noble gases, these gases consist of single atoms rather than molecules, but IMFs still play a crucial role. While noble gases are nonpolar and do not engage in traditional dipole-dipole interactions or hydrogen bonding, they do exhibit dispersion forces, which affect their boiling points. Understanding the types and strengths of intermolecular forces helps to explain why certain substances, such as noble gases, have specific boiling points.

Dispersion forces, also known as London forces, are a type of intermolecular force that is present in all atoms and molecules, regardless of their polarity. They come into play when there are temporary, spontaneous fluctuations in the electron distribution within atoms or nonpolar molecules, creating a momentary dipole.

In noble gases, these temporary dipoles induce corresponding dipoles in adjacent atoms, leading to a mutual attraction. The strength of dispersion forces is largely dependent on the size of the induced dipoles, which in turn relates to the number of electrons in the atoms and the size of the electron cloud. Larger atoms with more electrons can have stronger, more easily polarized electron clouds, allowing for more significant dispersion forces. Consequently, the increased strength of these forces in heavier noble gases contributes to their higher boiling points compared to the lighter ones.

The periodic trends in the properties of elements are patterns that occur within the periodic table and include variations in size, electronegativity, ionization energy, and more. For noble gases, a notable trend is the increase in atomic and molecular size as well as electron number as you move down the group.

This trend is particularly important when considering boiling points. As we move from helium (He), with the smallest atomic size and fewest electrons, down to xenon (Xe), which has a significantly larger atomic size and more electrons, the dispersion forces within these atoms become stronger. The increasing polarizability of the electron clouds due to the increasing number of electrons explains the ascending order of boiling points among noble gases. This correlation of periodic trends with physical properties like boiling points provides a fundamental explanation of why heavier noble gases boil at higher temperatures than their lighter counterparts.