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The chemical element with atomic number eighteen. Argon is a colourless noble gas. It can provide a protective atmosphere around hot, reactive materials, for example in welding, where excluding oxygen limits oxidation. Commercial production separates it by distillation of liquid air.^[ref]

Its discovery shows why a small disagreement can matter more than a reassuringly repeatable result. Two preparations can each give consistent measurements while producing different mixtures. The Rayleigh-Ramsay argon density experiment investigated just such a discrepancy. Gas density links the weighing to a mean molar mass; a separate separation process tests which component accounts for it.

Helium is the chemical element with atomic number 2 and symbol $\mathrm{He}$. It is a noble gas. It has a closed-shell electronic structure and is exceptionally unreactive. At ordinary pressure it remains liquid to very low temperatures without freezing; helium-4 also provides the classic example of a superfluid at sufficiently low temperature.^[ref]

Krypton is the chemical element with atomic number 36 and symbol $\mathrm{Kr}$. It is a noble gas. It is a monatomic gas under ordinary conditions and occurs as a minor component of air. Electrical excitation produces characteristic emission lines, connecting its atomic levels with lighting and spectroscopy.^[ref]

Radon is the chemical element with atomic number 86 and symbol $\mathrm{Rn}$. It is a noble gas. All its isotopes are radioactive. Radon-222 is formed in the uranium decay series and can leave the mineral containing its solid parent because it is a gas, linking nuclear decay with transport through pores and buildings.^[ref]

Xenon is the chemical element with atomic number 54 and symbol $\mathrm{Xe}$. It is a noble gas. Although relatively unreactive, it forms compounds such as xenon fluorides. Its chemistry demonstrates that a filled valence shell does not imply absolute inability to form chemical bonds.^[ref]