Occurrence and recovery

Key points: The noble gases are monatomic; radon is radioactive. Because they are so unreactive and are rarely concentrated in nature, the noble gases eluded recognition until the end of the nineteenth century. Indeed, Mendeleev did not make a place for them in his periodic table because the chemical regularities of the other elements, on which his table was based, did not suggest their existence. However, in 1868 a new spectral line observed in the spectrum of the Sun did not correspond to a known element. This line was eventually attributed to helium, and in due course the element itself and its congeners were found on Earth. The noble gases were given names that reflect their curious nature; helium from the Greek helios for sun, neon from the Greek neos for new, argon from argos meaning inactive, krypton from kryptos meaning hidden, and xenon from xenos meaning strange. Radon was named after radium as it is a product of the radioactive decay of that element. Helium atoms are too light to be retained by the Earth’s gravitational field, so most of the He on Earth (5 parts per million by volume) is the product of -emission in the decay of radioactive elements. High He concentrations of up to 7 per cent by mass are found in certain natural gas deposits (mainly in the USA and east ern Europe) from which it can be recovered by low-temperature distillation. Some He arrives from the Sun as a solar wind of particles. Neon, Ar, Kr, and Xe are extracted from liquid air by low-temperature distillation. The elements are all monatomic gases at room temperature. In the liquid phase they form low concentrations of dimers that are held together by dispersion forces. The low boiling points of the lighter noble gases (Table 18.1) follow from the weakness of these forces between the atoms and the absence of other forces. When helium (specifically 4He, not the rarer isotope 3He) is cooled below 2.178 K it undergoes a transformation into a second liquid phase known as helium-II. This phase is classed as a superfluid, as it flows without viscosity. Solid He is formed only under pressure.

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مواضيع ذات صلة

Other compounds of noble gases

Coordination compounds

Organoxenon compounds

Xenon oxygen compounds

Reactions of xenon fluorides

Synthesis and structure of xenon fluorides

Hydrogen peroxide

Water

Selenium, tellurium, and polonium

Sulfur

Reactivity of oxygen

Oxygen