Noble gas

The noble gases constitute Group 18 of the periodic table and comprise helium, neon, argon, krypton, xenon, radon, and the synthetic element oganesson. Characterised by exceptionally low chemical reactivity and distinctive physical properties, they have long played a central role in the development of atomic theory and practical applications involving inert atmospheres, lighting, refrigeration, and advanced scientific research. Under standard conditions, the first six noble gases exist as colourless, odourless, monatomic gases with very low boiling points. Their apparent lack of reactivity stems from the stability of their electron configurations, which feature complete valence shells.
Although traditionally regarded as perfectly inert, the noble gases have been shown to form compounds under specific conditions, leading to a revised understanding of their chemistry. Their history, physical characteristics, and uses illustrate their substantial scientific significance, as does the discovery of new members of the group through both natural processes and synthetic methods.

Chemical Inertness and Electron Structure

The hallmark of noble gas behaviour is their minimal tendency to engage in chemical reactions. Their valence electron shells are full—containing two electrons in helium and eight in the remaining elements—producing an energetically stable configuration resistant to further bonding. This stability explains why noble gas compounds are comparatively rare and often require extreme conditions to form.
Only a few hundred noble gas compounds have been identified, largely involving xenon, krypton, and radon. The relative inertness of these elements makes them desirable in contexts where unintended chemical activity must be prevented. For example, argon is widely used in welding and metal production as a shielding gas, while helium and neon serve as highly effective refrigerants owing to their exceptionally low boiling points. Helium’s buoyancy and safety properties make it suitable for lifting gases in airships and balloons.
The weak interatomic forces between noble gas atoms, mainly London dispersion forces, account for their very low melting and boiling points. These forces also explain why noble gases remain monatomic and gaseous under normal conditions, even when their atomic masses exceed those of elements that commonly exist as solids.

Discovery and Historical Development

The identification of noble gases unfolded over more than a century. In 1868, Pierre Janssen and Joseph Norman Lockyer detected a previously unknown spectral line while observing the solar chromosphere, naming the associated element helium after the Greek word for the Sun. Although the element was not isolated on Earth until later, its discovery demonstrated that spectroscopy could reveal new elements.
Earlier, in 1784, Henry Cavendish had detected a small component of air that was less reactive than nitrogen, though its significance was not recognised at the time. The breakthrough came in 1895 when Lord Rayleigh observed density differences between atmospheric nitrogen and chemically prepared nitrogen. Working with William Ramsay, he isolated a new gas, argon, named after the Greek term for “idle”. This discovery prompted a search for an entire class of previously unidentified gases.
Ramsay subsequently separated helium from the mineral cleveite, and by 1898 he had isolated krypton, neon, and xenon using fractional distillation of liquid air. Radon, identified in 1898 by Friedrich Ernst Dorn, was later recognised as a member of the group. The scientific community quickly appreciated the importance of these discoveries for atomic theory, and both Rayleigh and Ramsay received Nobel Prizes in recognition of their work.
The inclusion of the noble gases in the periodic table was confirmed when Dmitri Mendeleev accepted the evidence for their existence and placed them into a new group. Their discovery influenced key developments such as Niels Bohr’s model of electron shells and Gilbert N. Lewis’s octet rule, which clarified why atoms with full valence shells tend to be chemically stable.
A major milestone occurred in 1962 when Neil Bartlett synthesised xenon hexafluoroplatinate, the first noble gas compound. This breakthrough dispelled the belief that noble gases were entirely inert and opened the field of noble gas chemistry.

Production and Industrial Applications

Most noble gases are extracted from the atmosphere using industrial processes such as liquefaction and fractional distillation. Argon, neon, krypton, and xenon are separated from liquid air, while helium is typically obtained as a by-product of natural gas extraction, particularly in regions with high concentrations of subsurface helium. Radon, in contrast, is sourced from the radioactive decay of elements like radium, uranium, and thorium.
These gases have a wide array of practical uses.

• Argon is essential in welding, metal fabrication, and incandescent lighting due to its inertness.
• Helium is crucial for cryogenics, scientific instrumentation, and aviation applications.
• Neon is used in lighting and refrigeration.
• Krypton and xenon are valuable in lighting, lasers, and specialised imaging technologies.
• Radon, though radioactive, has historically been used in certain medical treatments.The extremely low reactivity of noble gases frequently underpins their industrial roles, ensuring stability and preventing unwanted chemical changes.

Oganesson and Modern Developments

Oganesson, the seventh member of group 18, is a synthetic element created in 2006 by bombarding californium with calcium. Only a few atoms have been produced, each with a half-life of less than a millisecond. While IUPAC classifies it within the noble gases, theoretical studies suggest that relativistic effects significantly alter its behaviour. It may be a solid under standard conditions and far more reactive than its lighter counterparts, raising questions about whether it fits the functional definition of a noble gas.
As research advances, oganesson remains an element of considerable scientific interest for understanding the limits of atomic structure.

Physical and Atomic Properties

Noble gases share a set of characteristic physical properties attributable to their minimal interatomic forces. Their melting and boiling points are extremely low, making them cryogenic under standard pressure. Helium is particularly unusual: it possesses the lowest boiling point of any known substance and cannot be solidified under normal atmospheric pressure. Its zero-point energy is sufficiently high that only high pressure can induce solidification, an effect explained by quantum mechanics.
The noble gases up to xenon possess multiple stable isotopes, while krypton and xenon each include naturally occurring radioactive isotopes in addition to stable forms. Radon is radioactive and decays rapidly, limiting its availability and increasing the need for controlled handling.
These properties have made noble gases essential in both theoretical research and applied science. From foundational studies in atomic structure to the creation of inert environments for delicate experiments, their unique characteristics continue to shape scientific understanding and technological innovation.