Noble-Gas Chemistry

Once thought wholly inert, the noble gases—above all xenon—form a real chemistry of fluorides, oxides, and related compounds, overturning the assumption that a full octet precludes reaction.

Definition

Noble-gas chemistry is the study of the compounds formed by the group 18 elements, principally the binary and oxofluorides of xenon, and of the bonding that allows these closed-shell atoms to combine with highly electronegative partners.

Scope

This topic covers the chemistry of group 18: the discovery and inertness of the noble gases, the conditions under which the heavier members react, the structures and bonding of xenon fluorides, oxides, and oxofluorides, the application of VSEPR to these molecules, and the more limited chemistry of krypton and radon. It treats the bonding and reactivity of noble-gas compounds rather than the spectroscopy used to discover the elements.

Core questions

• Why were the noble gases long considered chemically inert?
• What conditions allow xenon and krypton to form compounds?
• What are the structures and bonding of the xenon fluorides and oxides?
• Why does reactivity increase from helium toward radon?

Key concepts

• Inertness and ionization energy
• Xenon fluorides
• Xenon oxides and oxofluorides
• VSEPR geometry of noble-gas compounds
• Three-centre four-electron bonding
• Krypton and radon chemistry

Key theories

Reactivity of the heavier noble gases

The heavier noble gases have relatively low ionization energies and large, polarizable electron clouds, so sufficiently strong oxidizers such as fluorine and platinum hexafluoride can oxidize xenon to form stable compounds.

Structure and bonding of xenon fluorides

Compounds such as XeF2, XeF4, and XeF6 adopt geometries predicted by VSEPR from the lone pairs on xenon, with bonding describable by three-centre four-electron or molecular-orbital models without invoking d-orbital participation.

Oxides and oxofluorides

Hydrolysis and further reaction of the fluorides give xenon oxides and oxofluorides such as XeO3 and XeOF4, strong oxidizers whose existence further demonstrates a genuine and varied noble-gas chemistry.

Clinical relevance

Beyond their fundamental interest, noble gases are used as inert atmospheres, lighting and laser media, cryogens, and anaesthetics, while noble-gas fluorides serve as powerful fluorinating and oxidizing reagents.

History

The noble gases were discovered by Ramsay and colleagues at the close of the nineteenth century and were long held to be completely unreactive. Bartlett's 1962 preparation of a xenon–platinum fluoride compound shattered this belief and launched the systematic chemistry of the noble gases, especially the xenon fluorides and oxides.

Key figures

• William Ramsay
• Neil Bartlett
• Linus Pauling

Seminal works

• bartlett1962
• greenwood1997
• weller2018

Frequently asked questions

If noble gases have full octets, how can they react at all?

A filled octet makes reaction difficult but not impossible; the heavier noble gases hold their outer electrons relatively loosely, so extremely strong oxidizers such as fluorine and platinum hexafluoride can remove or share those electrons and form genuine chemical bonds.

Why is xenon far more reactive than helium or neon?

Ionization energy falls down group 18 as the valence electrons lie farther from the nucleus, so xenon's electrons are much easier to engage than helium's or neon's, which is why a stable chemistry exists chiefly for xenon and, to a lesser extent, krypton and radon.