Group 18 Elements : Noble Gases, Lecture notes of Chemistry

Download Group 18 Elements : Noble Gases and more Lecture notes Chemistry in PDF only on Docsity!

Group 18 Elements

: Noble Gases

Group 18 Elements : Noble Gases

 Group 18 consists of six elements: helium, neon, argon, krypton, xenon and radon.  All these are gases and chemically unreactive. They form very few compounds. Because of this they are termed noble gases  All the noble gases except radon occur in the atmosphere.  Their atmospheric abundance in dry air is ~ 1 % by volume of which argon is the major constituent.  Radon is obtained as a decay product of 226 Ra.

Atomic Radii : Atomic radii increase down the group with increase in atomic number Electron Gain Enthalpy : Due to stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

Physical Properties  All the noble gases are monoatomic.  Colourless, odourless and tasteless.  Sparingly soluble in water.  Have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces.  Helium has the lowest boiling point ( 4. 2 K) of any known substance

Chemical Properties In general, noble gases are least reactive due to the following reasons :

1. except helium ( 1 s 2 ) all have completely filled ns 2 np 6 electronic configuration in their valence shell. 2 .They have high ionisation enthalpy and more positive electron gain enthalpy

 But in 1962 , Neil Bartlett, then at the University of British Columbia, prepared a red compound which is formulated as O 2 + PtF 6

- .  He, then realised that the first IE of O 2 ( 1175 kJmol

• 1 ) was almost identical with that of Xe( 1170 kJ mol - 1 ).  He attempt and made same type of another red colour compound Xe

PtF 6

by mixing PtF 6 and Xe.  After this discovery, a number of xenon compounds mainly with most electronegative elements like fluorine and oxygen, have been synthesised Xe + PtF 6 XePtF 6 +^ Xe(PtF^6 ) 2 RT

• dependent on reactant ratio
• red-tinged yellow solid

XeF 6 can also be prepared by the interaction of XeF 4 and O 2 F 2 at 143K XeF 4

+ O

2

F

2 → XeF 6

+ O

2 Properties  XeF 2 , XeF 4 and XeF 6 are colourless crystalline solids and sublime readily at 298 K.  They are powerful fluorinating agents.

Properties  Hydrolysis : They are readily hydrolysed even by traces of water. For example : 2XeF 2 (s) + 2H 2 O(l) → 2Xe (g) + 4 HF(aq) + O 2 (g) 6XeF 4

+ 12 H

2 O → 4Xe + 2Xe 3

+ 24 HF + 3 O

2 XeF 6

+ 3 H

2 O → XeO 3

+ 6 HF

STRUCTURES OF THE XENON FLUORIDES sp^3 d Hybridization sp^3 d^2 Hybridization sp^3 d^3 Hybridization

Xenon-oxygen compounds XeO 3 XeOF 4 and XeO 2 F 2 Hydrolysis of XeF 4 and XeF 6 with water gives Xe0 3. XeO 3 6XeF 4 + 12 H 2 O → 4Xe + 2Xe0 3 + 24 HF + 3 O 2 XeF 6 + 3 H 2 O → XeO 3 + 6 HF XeO 3 is a colourless explosive solid and has a pyramidal molecular structure sp^3 Hybridization

Square pyramidal F F O O Xe sp^3 d^2 Hybridization ↑↓ See saw shape : sp 3 d Hybridization

Uses: Helium :  It is a non-inflammable and light gas. Hence, it is used in filling balloons for meteorological observations.  It is also used in gas-cooled nuclear reactors. Liquid helium (b.p. 4.2 K) finds use as cryogenic agent for carrying out various experiments at low temperatures.  It is used to produce and sustain powerful superconducting magnets which form an essential part of modern NMR spectrometers and Magnetic Resonance Imaging (MRI) systems for clinical diagnosis.  It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.