Chemistry at the Edge of the Periodic Table: The Importance of Periodic Trends on the Discovery of the Noble Gases and the Development of Noble-Gas Chemistry

Abstract

Initial attempts to form noble-gas compounds were prompted, guided, and illuminated by trends within the Periodic Table. The history of noble-gas reactivity commenced with Henri Moissan, who discovered elemental fluorine in 1886, and positioned F2 as the most reactive element in the Periodic Table by showing that it reacted with numerous metals and nonmetals, except the noble metals. Following on Sir William Ramsay and Lord Rayleigh’s discovery of argon, Moissan’s attempts in 1895 to react argon with F2 in an electric discharge failed. Upon Ramsay’s discovery of the remaining natural noble gases and his proper positioning of this new family of elements within Mendeleev’s periodic system, visionaries came forth who espoused the Periodic Table and noted trends within it that suggested the possibility of compound formation by krypton and xenon with, most notably, fluorine. A number of subsequent attempts to induce compound formation with these elements failed, but each attempt was steadfastly guided and refined with an eye to the Periodic Table. Neil Bartlett also looked to the Periodic Table and noted that the first ionization potentials of the noble gases followed a well-established trend of decreasing down a group and reasoned that he stood the best chance of oxidizing xenon with PtF6, a compound which Bartlett had already shown was a potent oxidizer by using it to oxidize O2 to form [O2]+[PtF6]−. The very similar first ionization potentials of O2 and Xe led Bartlett to attempt the reaction of PtF6 with xenon. These insights led to the synthesis of the first true noble-gas compound, “XePtF6,” in 1962. The key to unlocking noble-gas chemistry was a fluorine compound and it was soon shown that F2 could be directly reacted with xenon and krypton to form their fluorides which are the precursors to all known noble-gas compounds that are isolable in macroscopic amounts. Contemporary noble-gas chemistry continues to draw upon and further illustrate periodic behavior such as the stable oxidation states of xenon and krypton; structural analogies within the Periodic Table like those with Group 17 fluorides, oxide fluorides, and oxides; and the strong oxidative fluorinating properties of xenon compounds and, most notably, krypton compounds.