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NEET Chemistry Notes p-Block Elements – Group-18 Elements Noble gases

NEET Chemistry Notes p-Block Elements – Group-18 Elements Noble gases

 Group-18 Elements Noble gases

Group-18 Elements Noble Gases

Elements of 18 group are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) (radioactive). All of these are gases and chemically unreactive. Down the group their ionisation enthalpy decreases, therefore Xe reacts with oxygen and fluorine and forms different compounds. Xenon and radon are the rarest elements of the group. First prepared noble gas compound by Neil Bartlett in 1962 is XePtF6.

Occurrence

On account of their inert nature, the noble gases always occur in the free state. Argon is the most abundant noble gas in the atmosphere while radon is not present in atmosphere. He, Ar and Ne are also found as constituents of dissolved gas of certain spring waters.

Important Properties Physical Properties

  • Electronic configuration These gases have highly stable ns2np6 configuration.
    NEET Chemistry Notes p-Block Elements - Group-18 Elements Noble gases 1
  • Atomic and ionic radius On moving down the group, the atomic and ionic radii increases with increase in size.
  •  Boiling points On moving down the group, the boiling point increases with increase in size.
  • Ionisation energy and electron affinity These gases have stable ns2np6  (fully filled) configuration, thus, have no tendency to add or lose electron. Hence, their ionisation energy is very high and electron affinity is zero.
  • Heat of vaporisation and polarisability These gases possess very low values of heat of vaporisation due to the presence of very weak van der Waals’ force between their monoatomic molecules.
    However, the value of heat of vaporisation increases with atomic number. On moving down the group, the polarisability increases.
  •  Solubility Noble gases are slightly soluble in water and their solubility generally increases on moving down the group.
  •  Adsorption Except helium, all the noble gases are adsorbed by charcoal and the ease of adsorption increases down the group.
  • Liquification Ease of liquification increases down the group from He to Rn due to increase in intermolecular forces.
  • Conductivity Noble gases have high electrical conductivity at low pressure.
  • These gases are monoatomic gases (Cp/Cv = 1.667).

Chemical Properties

In general, noble gases are not very reactive.
Their inertness to chemical reactivity is attributed to the following reasons

  • The noble gases have completely filled ns2np6 electronic configurations in their valence shells.
  • The noble gases have very high ionisation energies.
  • The electron affinities of noble gases are almost zero.

Preparation and Properties of Some Compounds
NEET Chemistry Notes p-Block Elements - Group-18 Elements Noble gases 2

  • Partial hydrolysis of XeF6 gives oxylluorides, XeOF4 andXe02F2.
  • XeF2, XeF4 and XeF6 are colourless crystalline solids and sublime readily at 298K. They are powerful fluorinating agents.
  • XeF6 is extremely reactive. It cannot be stored in glass or quartz vessels as it readily reacts with Si02 present in glass.
  • Xe03 is a colourless, explosive solid and has a pyramidal shape. XeOF4 is a colourless, volatile liquid and has a square pyramidal shape.
  • XeF2 is linear and XeF4 is square planar. XeF6 has a distorted octahedral shape.

Uses

  • He is used in filling balloons for meteorological observations.
  • It is used in gas cooled reactor, to produce powerful superconducting magnets and as a diluent for oxygen in diving apparatus.
  •  Ne is used in discharge tubes and fluorescent bulbs.
  • Ar is used in filling bulbs and to produce inert atmosphere in various metallurgical operations.

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