p - Block Elements Part - 16 | Class 12th

 

Group 18 Elements

Group 18 consists of six elements- helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). All these are gases and chemically unreactive. They form very few compounds. Because of this they are termed noble gases.

 

ü  Occurrence: 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. Helium and sometimes 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 elements of the group.

Most abundant element in air is Ar. Order of abundance in the air is Ar > Ne > Kr > He > Xe.

ü  Electronic Configuration: All noble gases have general electronic configuration ns²np⁶ except helium which has 1s². Many of the properties of noble gases including their inactive nature are ascribed to their closed shell structures.

ü  Ionisation Enthalpy: They have very high ionization enthalpy because of completely filled orbitals. Ionisation enthalpy decreases down the group because of increase in size.

ü  Atomic radii: Increases down the group because number of shells increases down the group.

ü  Electron Gain Enthalpy: Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have larger positive values of electron gain enthalpy.

ü  Melting and boiling point: Low melting and boiling point because only weak dispersion forces are present.

ü  Chemical Properties: The reactivity of noble gases has been investigated occasionally ever since their discovery, but all attempt to force them to react to form the compounds were unsuccessful for quite a few years. In March 1962, Neil Bartlett, then at the University of British Columbia, observed the reaction of a noble gas. First, he prepared a red compound which is formulated as O₂⁺ PtF₆ ^– . He, then realised that the first ionisation enthalpy of molecular oxygen (1175 kj mol^–1) was almost identical with that xenon (1170 kJ mol^–1). He made efforts to prepare same type of compound with Xe⁺PtF₆^– by mixing PtF₆ and Xenon. After this discovery, a number of xenon compounds mainly with most electronegative elements like fluorine and oxygen, have been synthesised.

ü  Xenon-fluorine compounds: Xenon forms three binary fluorides, XeF₂, XeF₄ and XeF₆ by the direct reaction of elements under suitable conditions.

Xe (g) + F₂ (g)      673K, 1 bar            XeF₂(s)                                 

(xenon in excess) 

Xe (g) + 2F₂ (g)     873K, 7 bar       XeF₄(s)

(1:5 ratio)            

 

Xe (g) + 3F₂ (g)     573K, 60-70bar   XeF₆(s)

(1:20 ratio)          

 

 

XeF₆ can also be prepared by the interaction of XeF₄ and O₂F₂ at 143K.

XeF₄ +O₂F₂ →XeF₆ +O₂

XeF₂, XeF₄ and XeF₆ are colourless crystalline solids.

They are powerful fluorinating agents.

They are readily hydrolysed even by traces of water. For example, XeF₂ is hydrolysed to give Xe, HF and O₂.

2XeF₂ (s) + 2H₂O(l) → 2Xe (g) + 4 HF(aq) + O₂(g)

 

·       Structures: XeF₂ and XeF₄ have linear and square planar structures respectively. XeF6 has seven electron pairs (6 bonding pairs and one lone pair) and thus, have a distorted octahedral structure.

 

 

 

ü  Xenon – Oxygen Compound:

·       XeO₃: It is obtained by the hydrolysis of XeF₄ and XeF₆ with water. 6XeF₄ + 12 H₂O → 4Xe + 2XeO₃ + 24 HF + 3 O₂

         XeF₆ + 3 H₂O → XeO₃ + 6 HF

 

·       XeOF₂ andXeOF₄: Partial hydrolysis of XeF₆ gives oxyfluorides, XeOF₄ and XeO₂F₂. XeF₆ + H₂O → XeOF₄ + 2 HF

         XeF₆ + 2 H₂O → XeO₂F₂ + 4HF

                     XeO₃ is a colourless explosive solid and has a pyramidal molecular structure. XeOF₄ is a colourless volatile liquid and has a square pyramidal molecular structure.

 

 

ü  Uses of Noble Gases:

·       Helium is used in filling balloons for meteorological observations. It is also used in gas-cooled nuclear reactors. Liquid helium is used as cryogenic agent for carrying out various experiments at low temperatures. It is used as a diluent for oxygen in modern diving apparatus because of its very low solubility in blood.

·       Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes. Neon bulbs are used in botanical gardens and in green houses.

·       Argon is used to provide an inert atmosphere in high temperature metallurgical processes and for filling electric bulbs. It is also used in the laboratory for handling substances that are air-sensitive.

·       Xenon and Krypton are used in light bulbs designed for special purposes.