Group – 18 elements (inert gases)
Introduction :
→ The group 18 of periodic table consists of Helium (He). Neon (Ne). Argon (Ar). Krypton Kr). Xenon (Xe) and Radon (Rn). It is also known as group. All are non-metals. The last elements i…radon is radioactive in nature.
→ All the elements of group-18 are monostomic gases at room temperature. These gases do not have chemical reactivity at ordinary room temperature so they are called as inert gues. However Xenon and Krypton have been used to prepare a number of compounde.
→ It shows that these gars are not completly inert. Hence, now a days they are called as noble goes instead of inert gases. The name noble gases is analogous to noble metals (gold platinum) which show very low chemical reactivity.
→ As the gases of group-18 have very low abundance on earth so they are also called as rare gases. The elements are also termed as zerovalent elements because they show mro valenty due to chemical inertnes. They are also known as Aerogen whose meaning is “presence in air”.
Occurrence :
→ Except radon all the noble gases always occur in free state or elemental state in the atmophere. Their total percentage is about 1% by volume. In this percentage argon is about 0.83 which is the major component In natural gas the abundance of helium is about 2 to 7%. Helium with traces of neon sometimes hlso found in minerals of radioactive elements like pitch blende, clevite, monaite ete. Radon is radioactive hence, it does not occur in tree state because it decays very rapidly.
→ Helium, neon, argon also found in spring water. The relative presence of noble gases in atmosphere in terms of parta per million (ppm) is experied below:
Electronic Configuration of Group – 18 elements
→ The genral electronic configuration of noble gases is nonp, but the configuration of Helium is 13. From general electronic configuration it is clear that they have completely filled configuration which imparts the stability of noble gare.
→ Due to presence of fully filled configuration these elements neither have any tendency to gain electrons nor to lose electrons. That is why, these elements are completely inert. The electronic configuration of the elements of group-18 is given in table 7.21.
7.21 Electronic Configuration of Noble Gases
Isolation of Group – 18 elements
The isolation of noble uses are given below:
→ Helium : Commercially helium is isolated from natural gases. Natural gas mainly contains hydrocarbona (methane + hydrocarbons upto six carbon atoma), but varying amount of carbon dioxide, nitrogen, hydrogen sulphide and helium which is about 2-7%. For the isolation of helium naturals is compressed to about 100 atm and cooled upto 73 K Under these conditions except helium all the other gases get liquified. By this method about 99% pure helium is isolated.
→ Neon, Argon, Krypton and Xenon. These and can be obtained by the fractional distillation of liquid air During fractional distillation of liquid air O2, N2 and mixture of noble anses are obtained Prom the mixture of noble cases, individual noble gases can be isolated by adsorption over coconut charcoal at different temperatures
Radom Radon can be obtained by the disintegration or decay of Radium. The half life of radon is 13.82 days
Physical Properties of Group – 18 elements
The physical properties of elements of noble gases are summarized in table 7.22.
Table 7.22 Atomic and Physical Properties of Group 18 Elements
Some important atomic and physical properties of elements of group-18 are discussed below:
→ Physical Surte All the noble gases are monoatomic, colourless and odourless. They are monoatomic due to the presence of stable configuration (ns2p6 ) Their monoatomie nature can be proved by the following two facts.
- AL NTP, 22.4 L of each gas have weight equal to their a lomic mass in gram
- The ratio of their specific or molar heats at constant pressure and constant volume i.e. Cp/Cvis equal to 1.67.
→ Solubility. They are non polar in nature hence sparingly soluble in water. Their solubility increase on increasing their atomie number.
→ Atomic Radii : In period, it is observed that on moving left to right from group 1 to group-17 the atomic radii decreases gradually but the atomic radii of noble gases is exceptionally higher than the corresponding elements of group-17.
→ It is due to the fact that the atomic radii of noble gases correspond to van der Waal’s radii which are larger than covalent radii. Noble gases due to the presence of stable configuration cannot form covalent bond that is why they form only van der Waal’s bond. Atomic radii of halogen > Atomic radi of noble gases.
→ On moving down the group the atomic rudii of noble gases incerases gradually from He to Rn because the number of shells increases.
He < Ne < Ar < kr < Xe (Atomie Radi) Ionisation
→ Energy : In respective period, noble gases have the highest ionisation enthalpy due to the presence of stable configuration. On moving down the group ionisation enthalpy decreases due to increase in atomic radii and shielding effect of their inner electrons. He > Ne > Ar > Kr > Xe > Rn (Lanisation enthalpy)
→ Electron gain enthalpy or electron affinity : Due to the presence of stable configuration noble gases do not have tendency to accept additional electron. Therefore, their electron min enthalpy is positive because the electron will enter in an orbital of next energy shell and this process will require energy thus the process will become less positive because the size gradually increases from He to Rn.
→ Melting and Toiling Points: Due to presence od weak van der Waal’s forces of attraction betwen them they possess very low melting and boiling points in comparison to other elements of comparable atomic and molecular MAR But melting and boiling point increases with increase in atomic number because the van der Waals forces of attraction increase. Hence, on moving down the group m.p and b.p increes.
→ Thus helium has the lowest while radon has the highest melting and boiling point. m.p. and b.pa van der Waal’s forces of attraction atomic number
→ Heat of Fusion and Heat of Vaporisation : Generally, noble gases have very low value of enthalpy of fusion and enthalpy of vaporisation. But these enthalpies ince down the group on increasing Atomic number.
→ Ease of Liquefaction: The be of liquefaction depends upon the magnitude of the attractive foreer present between the atoms of molecules. The atoms of noble gases have weak vander waal’s force of attraction hence these cases cannot be liquefied easily. However, the magnitude of van der Waal’s forces of attraction increases on increasing atomie number hence on moving down the group the ease of liquefaction increase from He to Xe.
Chemical Properties of Group – 18 elements
In genral noble R Are not reactive, their chemical inertness is due to the following reason
- They have completely filled elettronic configuration i…ns np in their valence shell.
- They have very high value of ionisation enthalpie
- They have positive value of electron gain enthalpy.
- Therefore, noble m es neither have tendency to lose electron nor to gain electron. Due to above reason these elementa also do not enter into chemical combination
Discovery of Compounds of Noble Gases :
→ In 1962 Neil Bartlett observed that platinum hexafluoride (PtF6) which is a powerful oxidising agent reacts with dioxygen and yield an ionic solid dioxygen hexafluoroplatinate (V), O2+ [IPF6]–
O2(g) + PtF6(g) → O2+ [PtF6]– (s)
→ In above reaction PF6 oxidises O2 into O2+. Since O2 and xenon has some similarities such as:
- The first lonization enery of oxygen (1175 kJ/mol) is quite closer to the first ionisation erngy of xenon gas (1170 kJ/mol).
- The molecular diameter of oxygen and atomic radii of xenon are also quite close (≈ 4Å).
→ Thus, Bartlett thought that PtF6 should oxidise Xe into Xe+. Then, he mixed Xe with PtF6 a rapid reaction occurred and a red coloured compound Xe+ [PtF6]– was obtained.
→ After the discovery of this compound a number of compounds of xenon are prepared because the ionisation enery of xenon is lower than other noble gas elemente Xenon form compounds generally with fluorine and oxyben because they have small size and high electronegativity.
→ The compounds of Krypton are fewer. Only the fluoride of Krypton (KrF2) has been studied in detail Radon is radioactive hence its compounds are not known. Only RnF2 is identified which is used in radio tracer technique.
→ Some important and table compounds of Xenone with fluorine and oxygen are given in table 7.23.
Table 7.23: Some stable Compounds of Xenon
| Oxidation State | Compounds |
| + 2 | XeF2 (Xenon difluoride) |
| + 4 | XeF4 (Xenon tetra fluoride) XeOF2 (Xenon oxydifluoride) |
| + 6 | XeF6 (Xenon hexafluoride) XeO3 (Xenon trioxide) XeOF4 (Xenon oxytetrafluoride) XeO2F, (Xenon dioxydifluoride) |
| + 8 | XeO4 (Xenon tetra oxide) XeO3F2 (Xenon trioxydifluoride) |
Compounds of Xenon and Fluorine
Preparation : Xenon reacts with fluorine under appropriate condition and form xenon fluorides.
Physical Properties :
- XeF2, XeF4, and XeF6 are colourless crystalline soilda and they sublime readily at 298 K.
- XeF2 is the least volatile fluoride of xenon.
- Fluorides of xenon are strong fluorinating agents They hydrolysed easily even in the presence of moisture.
Chemical properties :
Reaction with water Xenon fluorides are easily hydrolysed even in trace of water. Different halides form different products with water.
XeF2 show slow hydrolysis and evolve O2
2XeF2 + 2H2O → 2Xe + 4HF + O2
XeF4 show violent hydrolysis and undergoes disproportionation reaction and form Xe and XeO3 highly explosive solid.
6XeF4 + 12H2O → 4Xe + 2XeO3 + 24 HF + 3O2
XeF6 also show violent hydrolysis and form highly explosive solid XeO3
XeF6 + 3HO → XeO3 + 6HF
Reaction with fluoride lon aceeptors : Xenon fluoride reacts with fluoride ion acceptors and forma cationic and anionie species.
Compounds of Xenon and Oxygen
Preparation : Oxides of Xenon can be prepared by hydrolysis of Xe4 and XeF6.
- 6XeF4 + 12H2O → 4Xe + 2XeO3 + 24HF + 3O2
- XeF6 + 3 H2O → XeO3 + 6HF
Oxyfluorides like XeOF4 are prepared by the action of anhydrous or cone. H2SO4 on barium perxenate.
B2[XeO6] + 2H2SO4 → XO4 2BASO4 + 2H2O
Properties :
- XeO3 is a colourless explosive solid.
- XeO4 is a colourless volatile liquid.
- XeO3 behave a strong oxidising agent in aqueous solution
- 6Pu3+ + XeO3 + 6H+ → 6Pu4+ + Xe + 3H2O
→ XeO3 reacts with base and farm monoalkali xenate, which slowly undergoes disproportionation to give xenon and perxenate.
Perxenate ion form yellow coloured solution and behave as powerful oxidising agents
Structure of Xenon Compounds
Xenon difluoride (XeF2)
- Hybridisation – sp3 d
- Geometry – Thangular bipyramidal
- Shape – Linear
Xenon tetra fluoride (XeF4)
- Hybridisation – sp3d2
- Geometry – Octahedral
- Shape – Square planar
Xenon hexafluoride (XeF6)
- Hybridisation – sp3d3
- Geometry – Octahedral
- Shape – Distorted octahedral
Xenon Oxydifluoride (XeOF2)
- Hybridisation – sp3 d
- Geometry – Trigonal bipyramidal
- Shape – T- shaped
Xenon oxy tetra fluoride (XeOF4)
- Hybridization – sp3 d2
- Geometry – Octahedral
- Shape – Square pyramidal
Xenon trioxide (XeO3)
- Hybridization – sp3
- Geometry – Tetrahedral
- Shape – Pyramidal
Xenon tetra oxide (XeO4)
- Hybridization – sp3
- Geometry – Tetrahedral
- Shape – Tetrahedral
Xenpn dioxydifluoride (XeO2F2)
- Hybridization – sp3
- Geometry – Trigonal bipyramidal
- Shape – Distorted trigonal bipyramidal
Xenon trioxydifluoride (XeO3F2)
- Hybridization – sp3 d
- Geometry – Trigonal bipyramidal
- Shape – Trigonal bipyramidal
Xenon dioxy tetrafluoride (XeO2F4)
- Hybridization – sp3 d2
- Geometry – Octahedral
- Shape – Octahedral
Use of Noble gases
Important use of Noble need are as follows
Helium :
- Helium gas is used in producing an inert atmosphere during the welding of aluminum and magnesium which can be oxidised easily
- It is used for food preservation
- Helium used in Alling the tube of aeroplane tyres.
- Liquid helium (b.p. 42 K) is used as cryogenic fluid to attain very low temperature.
- Helium in light and non-inflammable gas so it is used by deep en diverses the solubility of helium is very low in blood
- A mixture of 80% helium and 20% oxygen is used by deep sea divers as the solubility of helium in very low in blood
- Mixture of helium and oxygen is also used to siet breathing in asthma
- It is used as best transfer apent in gas cooled nuclear reactors as it has very high thermal conductivity and inert nature.
- Helium gas due to its inert nature is used as a filler in electric transformers.
- Helium gas is also used for research work for maintaining very low temperature.
Neon :
- Neon pas has a property to carry very high current under high voltage so it is used in safety devices for protecting the electrical instrumente like relays, rectifiers, voltmeters ete.
- It is used for filling sodium vapour lamps.
- Neon gas in used in Becon light as safty signal for air navigators because its light has power to penetrate fog.
- It is used in discharge tubes and fluorescent lamps for advertisement. In discharge tube neon gas produce an arrangered glow
- which can be seen at long distance even in fogs and mista. Il other gases are mixed with neon gas then it produces different
- colours known as neon signs. These neon signs are widely seed for advertisement.
- Neon bulbs are used in botanical gardens and creen house.
Argon :
- Pure argon gas in used in gas chromatography.
- Arron gas used in laboratory for handling substances that are air sensitive
- Argonas has inert nature so it is used for filling electric bulbs.
- It is used to provide inert atmosphere in high temperature metallurgical proces such as welding of metals or alloys.
- It is widely used for filling incandescent metal filament electric bulbs.
- Argon gas is also used for filling radio valves, rectifiers and fluroescent tubes.
Krypton and Xenon :
- Kr 85 is used to determine the thickness of metal sheets and plastic sheets
- Kr-85 is also used to regulate the voltage in electronic tubes.
- Mixture of Krypton and Xenon are used for filling incandescent metal filaments of electric bulbs
- Kr-Xe mixture is used in fash bulbs for high speed photography.
- Xenon is used for research purpose for detecting mssons
Radon :
- Radon is radioactive so it is used in radioactive research
- It is used for the treatment of cancer.
- Radon is used in X-ray photography to detect flows in metals and other solids.