We are again ready to go along chemistry 12th p- block elements as notes of group 18. We shall read and learn occurrences, properties and other behaviors of inert gases or noble gases present in group 18. This group includes six elements. They are Helium, neon, argon, krypton, xenon and radon. All these are gases and chemically inert. Some of them were found to form some compounds hence they are termed as noble gases.
Occurrences:
All the noble gases except radon occur in the atmosphere. Their atmospheric abundance in dry air is ~1% by Volume of which argon is major component. Helium and neon are found in minerals of radioactive origin e.g. pitchblende, monazite, cleveite. The main commercial source of helium is natural gas. Xenon and radon are the rarest element of the group. Radon is obtained as a decay product of 226Ra.
22688Ra → 86222Rn + 24He.
Atomic and physical properties of group 18 elements:
(1) Electronic Configuration: All noble gases have general electronic Configuration ns2np6 except helium which has 1s2.
(2) Ionisation enthalpy: The elements of group 18 show very high Ionisation enthalpy due to stable electronic Configuration. The Ionisation enthalpy of these elements decreases down the group with increase in atomic number.
(3) Atomic radii: Atomic radii increase down the group with increase in atomic number.
(4) Electron gain enthalpy: The elements of this group have no tendency to accept electrons hence they have positive value of electron gain enthalpy. Neon has highest positive electron gain enthalpy.
(5) Physical properties: All the noble gases are monoatomic. They are colourless, odourless and tasteless. They are sparingly soluble in water. They have very low m.p. and b.p. because interatomic interaction is weak dispersion forces.
Chemical properties of group 18 elements:
Noble gases are least reactive due to following reasons-
(A) The noble gases except Helium (1s2) have completely filled electronic Configuration in their valence shell.
(B) They have high Ionisation enthalpy and more positive electron gain enthalpy.
How the first compound was prepared :
Neil Bartlett was the first scientist to prepare a red compound which has formula as O2+PtF4–. He then realised that the first Ionisation enthalpy of molecular oxygen (1175kJmol-1) was almost identical with that of xenon (1170kJmol-1). He made efforts to prepare same type of compound of Xe and was successful to prepare another red Colour compound Xe+PtF6– by mixing PtF6 and xenon. After this discovery, a number of xenon compounds with Fluorine and Oxygen have been synthesized. The compound of krypton is only KrF2.
Xenon fluoride compounds:
Xenon fluoride compounds: Xenon forms three binary fluorides, XeF2, XeF4 and XeF6 by the direct reaction of xenon and fluorine under appropriate conditions.
Xe(g) + F2(g) → XeF2, 673K,1bar
(xenon in excess)
Xe(g) + 2F2(g) → XeF4, 673K,1bar
(1:5 ratio)
Xe(g) + 3F2(g) → XeF6 673K,1bar
(1:20 ratio) All the fluorides are solid.
XeF6 can also be prepared by the interaction of XeF4 and O2F2
XeF4 + O2F2 → XeF6 + O2
XeF2, XeF4 and XeF6 are colourless Crystalline solids and sublime easily at 298K. They are powerful fluorinating agents.
Hydrolysis of xenon fluorides :
They are easily hydrolysed even by traces of water.
2XeF2 + 2H2O → 2Xe + 4HF + O2
6XeF4 + 12H2O → 4Xe + 2XeO2 + 24HF + 3O2
XeF6 + 3H2O → XeO3 + 6HF
XeF6 + H2O → XeOF4 + 2HF
XeF6 + 2H2O → XeO2F2 + 4HF
Fluorinating behaviour of xenon fluorides:
Xenon fluorides react with fluoride ion acceptors to form cationic species and fluoride ion donors to form fluoroanions.
XeF2 + PF5 → [XeF]+ [PF6]–
XeF4 + SbF5 → [XeF3]+ [SbF6]–
XeF6 + MF → M+ [XeF7]– (where M= Na, K, Rb or Cs).
Structure of the compounds of Noble gases :
Application of inert gases:
Helium is 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 finds use as cryogenic agent for carrying out various experiment at low temperature. It is used to produce and sustain powerful super conducting magnets which form an essential part of modern NMR spectrometers and MRI systems for clinical diagnosis.
Neon is used in discharge tubes and fluorescent bulbs for the purpose of advertisements display purposes. Neon bulbs are used in botanical gardens and in green houses.
Argon is used mainly to provide an inert atmosphere in high temperature metallurgical purposes and for filling electric bulbs. There are no significant uses of xenon and krypton. They are used in light bulbs designed for special purpose.