Noble Gases

1. Noble Gases

2. Noble Gases The noble gases • helium (He) • neon (Ne) • argon (Ar) • krypton (Kr) • xenon (Xe) • radon (Rn) • form group 18 of the periodic table

3. Noble Gases Ooccurrence • Minor constituents of the atmosphere • Isolated first by Ramsay • Fractionation of liquid air • Helium occurs in radioactive mineral • Radon • Radioactive with short half-lives • Characterized in the decay series from uranium and thorium

4. Noble Gases Electonic Configurations • He 1 s2 • Ne [He] 2 s2 2 p6 • Ar [Ne] 3 s2 3 p6 • Kr [Ar] 3 d10 4 s2 4 p6 • Xe [Kr] 4 d10 5 s2 5 p6 • Rn [Xe] 4 f14 5 d10 6 s2 6 p6

5. Noble Gases The chemical inertness • The stability of the noble gases with respect to loss or acceptance of electrons is due to their high ionization potentials and the highly positive values of their electron affinities. These effects are essentially responsible for the chemical inertness of these elements.

6. Noble Gases Compounds • It was long believed that the noble gases were incapable of forming chemical compounds. • In 1962 three groups succeeded independently in preparing noble gas compounds. • In June 1962 Bartlett prepared the orange-yellow, moisture-sensitive compound "xenon hexafluoroplatinate" by reaction of Xe with PtF6 . • In July 1962 Hoppe obtained the first binary compound of xenon, XeF2 , • This was followed one month later by the synthesis of XeF4 by Claasen, Selig, and Malm.

7. Noble Gas Bonding in Compounds • A surprising result • The valence compounds of krypton and xenon do not involve a new type of chemical bonding. • The atoms are bound in the same manner as in the long-known interhalogen compounds, such as IF7 , and in TeF6 .

8. Noble Gas Bonding in Compounds • For the formation of noble gas compounds • An electron must be promoted from the p shell to the d shell • Formation of bonding electron pairs with another atom can occur with hybridization of the type s p d n (n = 1, 2, 3, 4).

9. Noble Gas Types of Compounds • Noble gas compounds can be divided into three general types : • Short-lived molecules containing noble gas atoms • Valence compounds • Inclusion compounds (clathrates and intercalation compounds)

10. Noble Gas Types and Stability of Compounds • Valence compounds • Only formed with the most electronegative elements (till now, F, Cl, Br, N, and C) • Whereby thermodynamically stable compounds with fluorine are formed only by xenon and radon.

11. Noble Gas Types and Stability of Compounds • The thermodynamic stability of noble gas compounds, for example, the halides, follows the following general rules : • The stability of the compounds EX2 increases with increasing atomic number of the noble gas and with decreasing atomic number of the halogen : • ArF2 < KrF2 < XeF2 < RnF2 • XeF2 > XeCl2 > XeBr2 • The stability of the compounds decreases as the oxidation state of the noble gas increases.

12. Noble Gas Compounds • Up to now, attempts to react helium, neon, and argon with other elements have failed • The chemistry of krypton is limited to the detection of the ions KrF+, Kr2F3+, KrF2 radical, and the synthesis of KrF2 and its complexes KrF2· 2 SbF5 and KrF2· x AsF5 . A report of the detection of KrF4 proved to be erroneous. The existence of a compound with a Kr – N bond is claimed

13. Noble Gas Compounds • Compounds of xenon are known in the oxidation states II – VIII, some of which are remarkably stable. Xenon (II) fluoride is even commercially available.

14. Table 21-2

15. Noble Gas Preparation of Compounds • Xe + F2 = XeF2 400 ºC, 0.1 M Pa, deficiency of F2 • Xe + 2F2 = XeF4 600 ºC, 0.6 M Pa Xe : F2 =1 : 5 • Xe + 3F2 = XeF6 300 ºC, 6 M Pa

16. Noble Gas Properties of Compounds • Hydrolysis XeF2 + 2OH- = Xe + 1/2O2 + 2F2 + H2O XeF4 + 6H2O = XeO3 + 2Xe + 3/2O2 + 12HF XeF6 + 3H2O = XeO3 + 6HF • Oxidation NaBrO3 + XeF2 + 2H2O = NaBrO4 + 2HF + Xe • Fluoridation 2XeF6+ 3SiO2 = 2XeO3 + 3SiF4

17. Noble Gas Molecular Structures of Compounds • The structures of the fluorides, oxyfluorides, and oxides of xenon follow the rules of the valence shell electron pair repulsion model (VSEPR) • When the lone pairs are taken into account • XeF2 , XeOF2 , and XeO2F2 have trigonal bipyramidal structures with a linear F – Xe – F axis in the gas phase.

18. Noble Gas Molecular Structures of Compounds • In XeF4 and its oxyfluorides, the four fluorine atoms occupy equatorial positions, while the electron lone pairs or oxygen atoms occupy the axial positions of the octahedral structure.

19. Molecular structure of XeF6 A) In the gas phase : dynamic, distorted octahedral ;

20. Molecular structure of XeF6 B) In solution : In nonbridging solvents (CF2Cl2 , CF3Cl, SO2ClF, F5SOSF5), XeF6 forms Xe4F24 tetramers by fluoride bridging, and perhaps also through weak Xe – Xe interactions. The 24 F atoms are magnetically equivalent, suggesting that they are involved in a rearrangement process ;

21. Molecular structure of XeF6 C) In the crystal : The tetrameric Xe4F24 units are "frozen" in the solid state and are best described as (XeF5+F–)4

22. Uses • Argon and helium are used in the welding, cutting, and spraying of metals; used in metallurgy as a protective gas. • Neon: high-voltage tubular lamps • Argon: mixture with nitrogen, used as filler gas for conventional light bulbs

23. Uses • Krypton: used as a better filler gas for high-quality light bulbs, also in halogen lamps • Xenon: gas-discharge lamps, are used as filler gases for lamps, sometimes as constituents of gas mixtures • High-purity gases are required for these applications.

24. Uses • Helium: (1997 Europe) • Low-temperature technology 36 % • Welding, cutting 14 % • Optical fibers 8 % • Breathing mixtures, diving 6 % • Analysis 14 % • Leak detection 9 % • Balloons 7 % • Other uses 6 %