What Is a Noble Gas? Elements That Mostly Refuse to React

Chemical bonding involves atoms sharing or transferring electrons to reach arrangements that are lower in energy, and these elements already have such an arrangement, with their outer electron shells complete as they stand. There is therefore nothing to gain from bonding, which is the whole explanation. That observation works in reverse and is the more useful direction, since the reactivity of every other element can be described in terms of how far it is from one of these arrangements and what it must gain or lose to reach one, which is the organising principle taught in every introductory course. The stability is not absolute, since the heavier members hold their outer electrons less tightly and do form compounds, but the pattern is strong enough to have made the entire column invisible to early chemistry.

The discovery sequence is unusual and each element came from a different route:

• •Helium was identified in the spectrum of the sun in 1868 before being found on Earth, which is the only element discovered that way
• •Argon was found in 1894 from a persistent discrepancy between the density of nitrogen from air and from chemical sources
• •Neon, krypton and xenon followed within a few years from fractional distillation of liquefied air
• •Radon was identified as a radioactive product of radium decay
• •The whole column had to be added to the periodic table, which had not predicted it
• •That addition was a substantial test of the table, which accommodated the new group without disruption

The claim that these elements form no compounds was believed absolutely until 1962, when a chemist reasoning about ionisation energies predicted that xenon should react with a sufficiently aggressive oxidising agent and demonstrated it, producing the first such compound. Xenon chemistry expanded rapidly afterwards and a substantial range of its compounds is now known, along with a smaller number for krypton and radon. Argon compounds have been made under extreme conditions. Helium and neon remain essentially inert, holding their electrons too tightly for any known reagent. The episode is a standard example of an established impossibility that was never actually tested, since the prediction required only reasoning about published numbers and the experiment was straightforward once anyone tried it, and the textbooks had been asserting the impossibility for six decades.

The sources of these elements are unrelated to each other and explain the very different economics. Helium is produced by radioactive decay deep in the Earth, accumulates in some natural gas reservoirs and is extracted as a byproduct of gas production, which means supply depends on gas extraction from particular fields rather than on demand, and the gas escapes to space once released so the resource is genuinely finite. Argon, neon, krypton and xenon are present in the atmosphere and are obtained by liquefying air and separating the components by boiling point, which makes argon cheap since it constitutes nearly one percent of air and makes xenon expensive since it is present in parts per ten million. Radon is produced continuously by decay of radium in rock and accumulates in buildings, where it is a significant cause of lung cancer and is managed by ventilation.

The applications follow directly from the unreactivity and from specific physical properties. Argon fills the space in welding and in metal production to exclude oxygen and nitrogen from hot metal, and it fills double glazing and preserves documents and food. Helium is used for lifting, for cooling superconducting magnets in medical scanners and research equipment, for leak detection and in breathing mixtures for deep diving, and its supply is a genuine concern since it escapes the atmosphere entirely once released and is obtained as a byproduct of natural gas extraction. Neon and the others produce the coloured light in discharge tubes, with each gas giving its own colour. Xenon supplies intense light in specialised lamps and is used as an anaesthetic. Krypton and xenon fill certain lamps and windows.

Complete outer electron shells mean there is nothing to gain from bonding, and the reactivity of every other element is described by how far it sits from that arrangement. The whole column went undiscovered until the 1890s and had to be added to the periodic table. The impossibility of compounds was asserted for six decades and disproved in 1962 by reasoning from published numbers.