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Chapter_17_Notes - Noble gases(group 18 He Ne Ar Kr Xe Rn...

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1 Noble gases (group 18) Shriver, Chapter 17 He, Ne, Ar, Kr, Xe, Rn All are characterized by extremely stable closed-shell electronic structures (ns 2 np 6 ) and high ionization enthalpies: He 2372 kJ/mol Ne 2087 kJ/mol Ar 1520 kJ/mol (less than F 2 ) Kr 1351 kJ/mol Xe 1170 kJ/mol similar to O 2 All are low-boiling gases. In the condensed state, they are held together by weak van der Waals forces (in this case, London forces) proportional to their polarizability. The boiling point of the least polarizable noble gas, He, at 4.2 K is the lowest of any known substance (compare to H 2 at 20 K). Discovery and uses Helium (named after the Greek helios , sun) was discovered in 1868 during a solar eclipse, as an emission line in the solar spectrum. It is the 2nd most common element in the universe (23%) after H (76%). On Earth, it originates in the α -decay of 238 U (an α -particle is a He nucleus; it becomes He by oxidizing other compounds). Major uses for He are in weather balloons, arc welding, as a dive gas, and as a cryogen for superconducting magnets. Argon (from the Greek word argos for inactive), neon (from the Greek word neon for new), krypton (from the Greek word kryptos for concealed) and xenon (from the Greek word xenos for strange) were all isolated in the late 19th century by low temperature distillation of air. Ar is 1% of air by volume and is the cheapest noble gas. It originates in the electron capture decay of radioactive 40 K. Its main uses are as an inert atmosphere in high temperature metallurgical processes and in incandescent bulbs. Neon is used in discharge tubes, for its bright red emission in the presence of an electrical current. Xenon is useful in solid-state NMR studies ( 129 Xe, I=1/2). Radon was discovered in 1902 from radium solutions. It occurs naturally as a product of the nuclear decay of uranium, and causes lung cancer by α -decay when inhaled. The chemistry of the noble gases is very limited; there is no chemistry (yet) of He and the first stable compound of Ar was made only recently. Xenon has the most chemistry; Kr is much less reactive; it is known primarily as KrF 2 . (Although Rn is presumably more reactive than Xe, its radioactivity makes it very difficult to study.) The first covalent compound of Ar was recently reported (Nature 2000, 406, 874). It was made by low temperature photolysis of HF in an argon matrix:
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2 HF + Ar + h ν H-Ar-F There is a strong contribution of the ionic resonance form (HAr) + F - to the bonding. Fluorides of xenon Not surprisingly, the most stable noble gas compounds are those with the most electronegative elements. Xe (5s 2 4d 10 5p 6 ) forms a series of fluorides, oxides and oxyfluorides. The stability of the fluorides of xenon is remarkable: XeF 2 mp 129°C XeF 4 mp 117°C Δ H f
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Chapter_17_Notes - Noble gases(group 18 He Ne Ar Kr Xe Rn...

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