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p-BLOCK ELEMENTS - NOBLE GASES

INTRODUCTION

The group 18 of the periodic table consists of colourless, odourless gases at room temperature, isolated by William Ramsay and lord Rayleigh in 1898 from air. They realized that a sample of  nitrogen formed synthetically had a different density than obtained from air after removing oxygen, water vapour and carbon dioxide. The sample from air was heavier and it was found to contain 1.0% mixture of inert gases by volume (except Rn). Radon was obtained by radioactive disintegration of radium.

Helium is second most abundant element in the universe.
Argon is the most abundant of all the inert gases in the atmosphere.

GENERAL CHARACTERISTICS

ELECTRONIC CONFIGURATION

Element
Symbol
At No.
Valence shell electronic confg.
Helium
He
2
1s2
Neon
Ne
10
[He] 2s22p6
Argon
Ar
18
[Ne] 3s23p6
Krypton
Kr
36
[Ar] 3d104s24p6
Xenon
Xe
54
[Kr] 4d10, 5s25p6
Radon
Rn
86
[Xe] 4f14, 3d10, 6s26p6

PHYSICAL STATE

They are all gases under ordinary conditions of temperature and pressure.

PRESENCE

Except Radon all occur in atmosphere in dry air 10% by volume. Main commercial source of helium is natural gas .

ABUNDANCE

In 1.0% air the abundance follows the order
Ar> Ne>He>Kr> Xe

ATOMICITY

The Cp/Cv =1.67 shows their monoatomic nature.

MELTING AND BOILING POINTS

Due to the increase in magnitude of Van der waals forces, the melting point and boiling point increases from He to Rn.

ATOMIC RADII

The atomic radii increases from He to Rn and it corresponds to the Van der Waals radii.

CRITICAL TEMPERATURE/CRITICAL PRESSURE

The critical temperature (Tc) and Critical pressure (Pc) of noble gases increase down the group
Element
He
Ne
Ar
Kr
Ye
Rn
Tc(K)
5.1
44.3
150.6
211.0
256.4
373.5
Pc(Atm)
2.26
26.86
47.99
54.3
58.2
62.4

DENSITY

The density of  noble gases increases down the group.

HEAT OF VAPORISATION

They have very low values of heat of vaporisation due to weak van der waals forces of attraction. The value increases down the group.

SOLUBILITY IN WATER

They are slightly soluble in water and solubility increases from He te Rn.

LIQUEFACTION

It is extremely difficult to liquefy inert gases due to weak van der waals forces of attraction among their molecules. Hence they possess low value of critical temperature also.

IONISATION ENERGY

All noble gases possess very stable (ns2 and ns2p6) electronic configuration. Therefore, ionisation energy of noble gases is very high and decreases down the group.

ELECTRON AFFINITY

Due to the presence of stable electronic configuration they have no tendency to accept additional electron. Therefore electron affinity is almost zero.

POLARIZABILITY

The polarizability increases with the size and hence follows the order.
He< Ne< Ar< Kr< Xe

ADSORPTION BY CHARCOAL

Except helium all are adsorbed by coconut charcoal at low temperature.  The extent of adsorption increases down the group.

DISCOVERY

  • Argon - It was isolated by Rayleigh and confirmed by Ramsay, from air (free from O2, moisture and CO2). Due to its inertness the gas was named as argon (meaning lazy).
  • Helium - It was observed in the spectrum of the sun hence name helium, from helios, which means sun, by Lockyer and Frankland. Ramsay obtained it from gases occluded in uranium minerals.
  • Neon, Krypton and Xenon - These were obtained by fractional distillation of liquid air under reduced pressure.  These were named as neon means “new” krypton means “hidden” and Xenon means “stranger or foreigner”.
  • Radon -  Spectroscopically was noticed by Dorn and isolated by disintegration of radium by  Rutherford and Soddy.

ISOLATION OF RARE GASES

The following methods are employed for the isolation of Rare gas mixture.

RAMSAY AND RAYLEIGH’S FIRST METHOD

Air (dry and free from CO2)
Residual gas - mixture of inert gases.

RAMSAY AND RAYLEIGH’S SECOND METHOD

(CO2 and NO2 are absorbed by NaOH and O2 if any is removed by alkaline pyrogallol)

FISCHER AND RINGE’S METHOD

CO2  is absorbed in KOH and residual gas is dried over Conc H2SO4 and P2O5

SEPARATION OF RARE GASES BY DEWAR’S METHOD

[He, Ne, Ar, Kr, and Xe] + Coconut charcoal at 171K
From liquid air - The difference in the boiling points of various constituents of liquid air make possible their separation by fractional distillation.
Element
He
Ne
N2
Ar
O2
Kr
Xe
B.pt (K)
4
27
77
87
90
121
124

HELIUM FROM OTHER SOURCES

He is also obtained by heating mineral clavite to 1273K. The gas evolved is collected over potash solution to free from any CO2. It may also be obtained from mineral  by heating with dil.H2SO4 or KHSO4.

PROPERTIES OF NOBLE GASES

The noble gases are inert in nature because of their completely filled s and p subshells. In 1962 the first compound of noble gases was prepared. It is hexafluoroplatinate prepared by Bartlett.
Now many compounds of Xe and Kr are known with fluorine and oxygen.
No compounds of He, Ne or Ar exist, except under very special conditions (very unstable, not neutral compounds).
Rn is known to react with fluorine but its radioactivity makes the study of its compounds difficult and dangerous. Kr forms one stable neutral molecule, KrF2.  A, Kr- N bond has also been reported, stable only below –50º C.
Xe forms several compounds with fluorine and oxygen and Xe-N and Xe-C bonds have been reported.

STRUCTURES OF XENON FLUORIDES

XeF2 hybridisation (sp3d)
Linear

XeF4 hybridisation (sp3d2)
square planar

XeF6 hybridisation (sp3d3)
Pentagonal pyramidal or distorted octahedral

STRUCTURES OF OXYFLUORIDES AND OXIDES OF XENON

XeO3 is the anhydride of Xenic acid H2XeO4

USES

  • Helium - It is non-inflammable. It has low density and its lifting power is 92% that of hydrogen and used to lift weather balloons and airships.  It is used as breathing mixture (or oxygen diluent) for divers. Mixture of O2 and He is used in the treatment of asthma. It is also used for inflating the tyres of aeroplanes.
  • Neon - When an electric current is passed through a sample of the gas, it has a characteristic orange - red glow. Neon lighting is used for advertising. The noble gases argon (purple), Xenon (blue green) and Krypton (pale violet) are also used in “neon” lighting.
  • Argon - It is used primarily to create an inert atmosphere in light bulbs, welding and fluorescent bulbs. It is also used in geiger counters, as it becomes ionised in the presence of radiation. The ratio of 40K to 40Ar can be used to date the age of rocks since argon is obtained by radioactive decay of an isotope of potassium.
  • Krypton - The light emitted by Krypton in an electric discharge tube is used for runway and approach lights in airports.
  • Xenon - It is used in electrical flash bulbs for high speed photography.
  • Radon - In radiotherapy of cancer.

CLATHRATES

A number of organic and inorganic compounds having noble gases trapped into the cavities of crystal lattices are called enclosure or clathrate compounds. They are known as cage compounds also.

The substance having cavities in crystal lattices is called the host and atom of noble gases entrapped in it is called the guest which are held by Van der waals forces of attraction. The clathrates are non stoichiometric compounds. When clathrates are heated or dissolved the guest atom escapes from the host.

He and Ne do not form clathrates due to their small size.

TYPES OF CLATHRATES

They are of two types.
  • Gas hydrates - Solid water having entrapped Ar, Kr, or Xe
  • Quinol Clathrates - Quinol having entrapped Ar, Kr, and Xe

USES OF CLATHRATES

  • Separation of noble gases - Since Ne does not form a clathrate with Quinol it is separated from Ar, Kr and Xe. The latter form a clathrate with quinol.
  • Xe- 133 clathrate is a source of ɣ- radiations
  • Kr - 85 clathrate is a source of β - radiations
  • As an anaesthetic - Xe clathrate is used for this
  • For transporting isotopes of noble gases.

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