Chemistry
Why Noble Gases Have Increasing Boiling Points Down the Group
Quick fact
The boiling point of helium is about 4.2 K, while radon’s is about 211 K. That is a fivefold increase in absolute temperature, even though all noble gases are nonpolar and unreactive.
Why this is interesting
Helium boils at -269°C, barely above absolute zero, while radon boils at -62°C — warm enough to be a liquid on a cold winter day. Why do these 'inert' gases have such different boiling points?
Read the full explanation
Understanding Why Noble Gases Have Increasing Boiling Points Down the Group
Noble gases exist as single atoms in the gas phase. To become a liquid, they must be cooled until their kinetic energy is so low that the weak attractions between atoms can hold them together. These attractions are called London dispersion forces, and they arise from temporary fluctuations in the electron cloud around each atom. A larger atom has more electrons, and those electrons are farther from the nucleus, making the cloud more easily distorted. This distortion creates a temporary dipole that induces a dipole in a neighboring atom, producing a net attraction. Because atomic radius increases down the group (He < Ne < Ar < Kr < Xe < Rn), the dispersion forces become stronger, so more energy (a higher temperature) is needed to break them during boiling. Thus, boiling points rise steadily down the group.
A deeper explanation
The trend is governed by polarizability—how easily an electron cloud can be distorted. For noble gases, polarizability increases with the number of electrons and the size of the atom. Down the group, the valence shell becomes more distant from the nucleus and electrons are less tightly held, so the atom’s electron cloud is more deformable. When two atoms approach, transient fluctuations create instantaneous dipoles that induce opposing dipoles in nearby atoms, producing an attractive force. The strength of this force scales with the product of the polarizabilities of the two atoms, so larger atoms experience much stronger attractions. Consequently, the enthalpy of vaporization increases, requiring a higher temperature to provide enough thermal energy to separate the atoms. This explanation is not unique to noble gases; it applies to all nonpolar molecules and explains why larger molecules in a homologous series often have higher boiling points. Understanding this trend in noble gases offers a clean, simple case of how a physical property reflects fundamental atomic properties.