Chapter_17_Notes_Noble Gas.pdf

1 Noble gases (group 18) Shriver, Chapter 17 He, Ne, Ar, Kr, Xe, Rn All are characterized by extremely stable closed-shell electronic structures (ns2np6) and high ionization enthalpies: He 2372 kJ/mol Ne 2087 kJ/mol Ar 1520 kJ/mol (less than F2) Kr 1351 kJ/mol Xe 1170 kJ/mol similar to O2 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 H2 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 238U (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 40K. 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 (129Xe, 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