He (Helium), Ne (Neon), Ar (Argon), Kr (Krypton), Xe (Xenon), Rn (Radon) belongs to 18th group in periodic table. As these elements found in air (except Rn), they are called "Aerogens". Their abundance in air is 1% by weight. Due to stable ns2 np6 octet (He = 1s2) configuration, they have high ionization enthalpy and positive electron gain enthalpy. They neither lose nor gain nor share the electrons and called "Noble (inert) gases". Due to inert nature of these elements, discovery of these elements took more than 100 years.
P.J.C. Janssen & J.N. Lockyer found a new element Helium (Helios = Sun) in 1868 during total solar eclipse. Henry Cavendish found that N2 separated from air 1 contain (th part by volume of N2) a gas "Argon" (Argoes = Lazy). Abundance 125 of Ar is maximum (0.934%) in air among other noble gases. Ramsay & Travers identified a gas "Neon" (Neos = New) during fractional distillation of liquid Argon. By repeated fractional distillation of liquid air, Ramsay found another gas "Krypton" (Krypton = Hidden). He also found "Xenon" (Xenon = Stranger) from Krypton.
Radon is obtained during the decay of Ra − 226.
88Ra226 → 86Rn222 + 2He4
Due to inertness, they are very useful. Helium is used in gas cooled atomic reactors (as a heat transfer agent). 'He' is used for creating inert atmosphere during welding of Mg & Al. O2 − He mixture (20% O2 + 80% He) is used in the treatment of Asthma and also for artificial respiration in deep sea diving (if air is used instead of 'He', N2 present in air dissolved in blood under high pressure and causes pain called "Caisson Sickness" or "bends"). 'He' is used as cryogenic agent (b.p.: 4.2 K) in NMR (Non Magnetic Resonance) spectrometers and MRI (Magnetic Resonance Imaging) systems used for clinical diagnosis. 'He' is non inflammable and light gas (but heavier than H2), used in filling baloons for meteorological observations.
Neon (Ne) bulbs are used in botanical gardens. Ne is used in fluorescent bulbs for advertisement display purposes. 'Ne' is used in warning signals (e.g.: air ports) as 'Ne' lights are visible from longer distances even in fog and mist. When 'Ne' is mixed 8elements-online.qxd 19/9 pb-1 with other gases, they produce different colours, used in Neon signs (in advertisements).
Argon (Ar) is used in filling fluorescent lamps and electric bulbs to increase the life of the filament (lamp). Ar is used to provide inert atmosphere during arc welding of metals and alloys.
Krypton is used in electric bulbs, in measuring thickness of metal sheets, Kr - 85 is used in electronic tubes.
Xenon is used in electric bulbs, in high speed photographs, in bubble chamber to identify neutral measons and gamma photons. Radon is radioactive and used in the treatment of cancer, in the detections of defects (cracks, void space) in metals and solids, in industrial radiography.
Noble gases are mono atomic (Cp/Cv = 1.67), odourless, colourless, inert.
They have very low M.P. & B.P. due to weak dispersion forces (Vanderwaals forces).
These gases are slightly soluble in water. Except Helium, all the noble gases are absorbed by activated coconut charcoal. Due to stable electronic configuration, they exhibit very high ionization enthalpies and positive electron gain enthalpies (earlier electron effinity of noble gases was taken as zero). Upto 1962 people thought that noble gases do not combine with other elements, so they does not form compounds. Xe has low ionization enthalpy (large atomic size) among the noble gases. In 1962 Neil Bartlett observed that PtF6 reacts with O2. O2 + PtF6 → O2
+[PtF6]−
'He' also observed that first ionization enthalpy of Oxygen (1175 K.J./ mole) is very closer to first ionization enthalpy of Xe (1170 K.J./ mole) and thought that PtF6 also reacts with Xe and prepared first noble gas compound XePtF6. Xe + PtF6 278 K → Xe+ [PtF6]−
Due to high electro negativity values of F & O, they can form compounds with noble gases (like Xe). Xe can form XeF2, XeF4, XeF6, XeO3, Xe04, XeO2F2 and XeOF4 compounds.
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