The periodic table arranges elements by atomic number and shows how their properties change periodically. By the late 19th century a whole group of extremely unreactive gaseous elements was still missing from this table. Using Lord Rayleigh’s density anomaly as a clue, Ramsay combined fractional distillation of liquid air with spectroscopic analysis to isolate and identify argon, helium, neon, krypton, xenon, and radon. From their atomic weights and chemical inertness he placed them in a new far-right column of the periodic table, closing an important gap in periodic law. Today noble gases are essential in fluorescent lamps, lasers, semiconductor processing, and even spacecraft propulsion, so Ramsay’s work has brought enormous benefits to society.

Ramsay first liquefied air and carried out stepwise fractional distillation, removing nitrogen and oxygen and concentrating the high-boiling residue. Introducing this residue into discharge tubes and recording its emission spectra, he detected unfamiliar line series that proved the presence of new elements. He determined their densities and atomic weights by methods such as the watch-glass technique and the Meyer method, and, noting their valence of zero, proposed a separate group in the periodic table. This work later underpinned the closed-shell electron configuration (ns2np6) concept and influenced Werner’s coordination theory and the rise of quantum chemistry. Moreover, the identification of terrestrial helium isotopes had implications for studies of Earth’s interior and solar system formation.

Ramsay employed a Hampson-type liquefier to condense air at ~80 K and executed controlled distillation between −186 °C and −62 °C, thereby enriching the noble-gas fraction. Emission spectra of the residue were obtained with a prism spectroscope over 300–800 nm, and previously unknown lines were identified with wavelength accuracy better than 0.1 nm. For argon he combined precision densimetry with gas-equivalent determinations to fix the relative atomic mass at 39.9, while helium was shown to be identical to the α-decay product. Neon, krypton, and xenon were quantified from distillation curves based on boiling points, and thermal conductivity and γ ratios at standard conditions confirmed their monatomic nature. In positioning these elements, Ramsay extended Mendeleev’s scheme, introducing a valence-zero Group 0 (later Group 18) and revising the period count. These data were later incorporated into Langmuir’s octet theory, Lewis’s electron-pair model and Pauling’s electronic configuration theory, steering the development of noble-gas and high-pressure chemistry.