B.sc 1st year Book
(Page 16)

Group Zero (18) Elements: He, Ne, Ar, Kr, Xe, Rn Noble gases

The elements of the zero group are also known as inert gases or rare gases. The elements helium, neon, argon, krypton, xenon, and radon constitute this group of the periodic table. Except for radon, all are present in atmospheric air. Radon is a radioactive element and is obtained as a disintegration product of radium. The name noble gases or inert gases or rare gases are misnomers. In 1962 after the discovery of xenon fluorides which show that it is not inert. The name noble gases are given due to their nonreactive nature. In the same way, as it is found in the case of noble metals which are reluctant to react and are found to be the least reactive metals. Rare gases exist between most electropositive metals of the IA group and the most electronegative group of halogens.

  1. Electronic Configuration of noble gases:

Their electronic configuration and percentage by volume in the air are given below:Table 4.18: Electronic configuration of zero group elements Helium has 2 electrons to complete its shell whereas other inert gases have 8 electrons in their outermost shell (ns 2np6 ). This is a very stable electronic configuration and is closely related to its chemical inertness. The electron affinity of all these metals is zero and they show very high values of ionization potential that is the highest among all other elements. They have the least tendencies to gain or lose electrons under normal conditions. All the elements are monoatomic and have the least tendencies to form bonds. However, the new research has shown that they perform chemical reactions with some specific elements under special conditions resulting in the formation of chemical compounds.

Table 4.19: Some physical properties of the elements of zero group

Elements He Ne Ar Kr Xe Fin
Atomic radii 1.22 1.60 1.92 1.98 2.18 2.20
B.P. (∘C) −269.00 −246.00 −186.00 −153.00 −108.10 −62.00
M.P. (∘C) −272.00 −248.60 −189.40 −157.20 −111.90 −71.00
(at26 atm.pre65)
L.P. in eV 24.60 21.60 15.70 14.00 12.10 10.80
Solubility per liter of water at 0∘C 9.70 11.40 58.00 110.00 240.00 ..
Note: He and Ne are most volatile hence they pass along with N2 from the fractionation of liquid air in the fractionation column.

2. Occurrence of noble gases:

The noble gases occur in traces in atmospheric air hence they are also called rare gases. The percentage of their presence in the atmosphere is given in above table 4.18.

(a) Natural gas :

The hydrocarbon gases which are evolved naturally from different wells such as natural gas from the well of Kansar of USA consist of about 7% of He.

(b) Minerals :

Helium gas is found in a variety of minerals like Clevite, monazite, and pitchblende Hillebrand. It is found in the pores of these ores. The gas evolved when these ores are heated above 1000∘C. The gas present in the pores of ores is believed to be alpha particles an origin of a radioactive substance.

(c) Sun’s Atmosphere:

First of all, He-gas was discovered by Jannsen during a total solar eclipse. In 1868 Lockyer also observed a bright yellow line near D1 and D2 lines of Na in the solar spectrum during a total solar eclipse. He observed a new yellow D3 line different from the D1 and D2 yellow lines of sodium, He named it helium on the basis of the Greek name of Sun ‘Helios’. It was found that the formation of He-gas is constantly taking place by the nuclear reactions at the sun chromophore. In this reaction, four hydrogen atoms are fused together and form one He atom with a loss of mass.
The mass of four H-atoms is 4.032 g and He-atom is so formed by the fusion has a mass equal to 4.003 g. Thus, during the fusion process, there is a loss of mass equal to 0.029 g This loss of mass is converted to energy on the basis of Einstein’s equation E= mo 2 where ‘ m ‘ is the loss of mass and ‘ c ‘ is the velocity of light. In this way, an enormous amount of light energy is continuously produced by the sun and emitted to the earth’s surface.

(d) Observation of Cavendish:

The atmospheric air consists of N2, O2, and CO2 gases along with a small amount of NH3 and NO which are formed by lighting or by the action of light by the combination of N2, O2, and H2. In the year 1784-1785 Cavendish isolated a mixture of inert gases which he named inert N2. During his experiment, he added an excess O2 gas into N2 gas in a closed tube with an electric spark, where both gases are combined together to form NO which is removed by absorbing it in NaOH or KOH solution.
N2+2O2⟶2NO2
The excess O2 gas present is removed as SO2 by bumping it with sulfur (S). SO2 is also absorbed by the KOH solution. In this way, Cavendish obtained 1/120th portion of the gas which refuse to combine with anything. He thought that it is inactive N2 and could not publish his work. After an interval of more than 100 years, Lord Rayleigh in 1895 again repeated the Cavendish experiment and determined the density of N2 as isolated from the atmosphere. He observed that the density of the residual gas is to be 0.47% higher than that of N2 gas prepared in the laboratory by the heat of decomposition of NH4O2.
NH4NO2 ⟶ N2+ 2H2O
Many investigators also observed it and ignored this discrepancy but Rayleigh did not ignore it and repeated the same experiment several times. Lastly, he concluded that N2 gas obtained from the atmosphere must contain some heavier gases which are entirely different from N2 gas. The density of chemical N2 was 1.25207 and that of residual atmospheric N2 is 1.25718. Rayleigh and Ramsay repeated the Cavendish experiment by applying an improved technique and obtained the left residual gas. They determined the vapor density (VD) of the gas and found it to be 20 with respect to hydrogen whose V.D. Is taken as 1. They also determined the VD of N2 on the same scale and found it 14. Thus, they concluded that the gas so obtained is inert. They named it “argon” on the basis of the Greek word ‘Argos‘ which means idle or inert.
Later on, Ramsay and Traverse suspected that the gas argon discovered earlier must be a mixture of members of similar constituent gases. They also confirmed the above evidence on the basis of spectroscopic studies. After this, they repeated their experimental work for several years during which they liquified the mixture of gases and collected them, and then carried out their fractional distillation. To their great surprise, they again isolated a new element which was named Neon (Greek word Neo means new). The identity of this gas was established by the study of its spectrum. They again determined the VD for this gas which was found to be 10.1 with respect to H2 whose VD is taken to be one. Thus, the atomic weight of Ne is confirmed as 20.2 (At wt =2× V.D).
Once again both the scientists started their work with a large volume of the residual gas and liquefied it. From the liquid, they carried out fractional distillation. During this, they were able to separate different fractions and isolate another new element which was named “Krypton“(Kr). The Krypton means hidden element. Lastly, after the successful discovery of krypton within a very short period they discovered the other noble gas and named it as “Xenon” (Xenon = stranger). The credit goes to the experimental skill of sir William Ramsay which led to the discovery of a complete group and studied their properties in detail. Lastly, Ramsay studied a was emanating from a radioactive element like Ra, Ac, and Th. The emanating gas was merely a tiny bubble. He studied all its properties and was given the name “niton” or “Radon” in 1902. The complete discovery of inert gases is also announced by Dom.
The role played by noble gases is most important in the development of the theories of chemical bonding. All the elements of the periodic table acquire an electronic structure of corresponding inert gas at the end of the period during the chemical combination. This concept explains the systematic description of their chemical behavior. If an atom attains n s 2p6 configuration in its outermost valence shell, it becomes extremely stable. Thus, natural stability arises when an atom acquires a noble gas configuration during the chemical combination resulting in the formation of a stable compound. This is the basis of the electronic theory of valency.

3. Isolation of inert gases from atmospheric air :

There are two methods involved in the isolation of liquid air.

Physico-chemical methods :

In this method, the atmospheric air is taken and excess O2 has added to it when the electric spark is passed through the mixture of gases. NO2 is formed by the combination of N2 and O2.
N2 + O2 ⟶ 2NO
2NO + O2 ⟶ 2NO2
2NO2 + 2NaOH ⟶ NaNO2 + NaNO3 + H2
The NO2 formed is absorbed by NaOH and excess of O2 is removed by bumping S in it, the SO2 so formed in this reaction is again absorbed by NaOH. The remaining gases are a mixture of inert gases. The removal of N2 and O2 is also carried out by passing these gases over a mixture of CaC2 and CaCl2 in the ratio of 9:1 in a retort at 800∘C. N2 forms calcium cyanamide as shown below-
CaC2 + N2 ⟶ CaNCN + C
2C + O2 ⟶ 2CO
The carbon converts atmospheric O2 into CO which is passed over copper oxide to convert it into CO2. This CO2 is absorbed by KOH. There is also the possibility of the formation of CaCO3 by CO2 when passed over CaC2.
CuO+CO⟶Cu+CO2
2CaC2+3CO2⟶2CaCO3+5C
The remaining mixture of inert gases is dried over conc. H2SO2 and finally over P2O5. Ramsay and Rayleigh improved the technique adopted by Cavendish as shown in figure (4.20). They took a large flask of 50-60 liter capacity and used two large Pt electrodes for passing an electric discharge through it with a continuous circulation of NaOH in the form of a fountain. An induction coil is used to pass an electric discharge at 6000−8000 volts between the Pt-electrodes. As a result, N2 is removed from the air by combination with O2 in this case, NO is formed which is then converted into NO2 in excess of O2, and NO2 is separated by dissolving it in conc.NaOH solution and finally we get a free form of a mixture of noble gases.

Ramsay and Rayleigh improved the technique adopted by Cavendish as shown in figure (4.20)
4. Separation of inert gas mixtures :

(i) Dewar’s method:

The mixture of inert gas was introduced into a bulb filled with coconut charcoal and placed in a Dewar’s flask to maintain the temperature at −100∘C (Fig. 4.21). The whole system is left for an hour to maintain the temperature. After attaining the temperature of −100∘C the charcoal absorbs Ar, Kr, and Xe whereas He and Ne remain unabsorbed. He and Ne are pumped out of the bulb and is placed in contact with another bulb at −180∘C.
The bulb containing absorbed Ne is warmed to get the No-gas.
The first charcoal containing Ar, Kr, and Xe is placed in contact with another third charcoal whose temperature is of liquid air. Ar goes to third charcoal by diffusion and is removed on warming it. First charcoal containing a mixture of Kr and Xe is warmed up to −90∘C at which Kr.
Dowar's coconut charcoal method for the separation of noble gases Flow Chart of Dewar's Method

5. Fractional Distillation of liquid air:

Noble gases are isolated by fractional distillation of liquid air. This method was developed by the scientist Raleigh and Ramsay after a few years of their discovery. The fractionation depends upon the difference in their boiling points.
Fig. 4.22: Separation of noble gases from the air
Ar and O2 have equal volatility power hence, it remains mixed with O₂. The least volatile gases are Kr and Xe hence, they were left behind when all other components are distilled off. Cold air is compressed at 10-35 atmospheric pressure and entered from the lower part of the apparatus. The cold air rises up with pipe B surrounded by liquid O2. In vessel B a part of air is liquefied and collected in vessel A (as shown in fig. 4.22) consisting of O2 and less volatile Ar, Kr, and Xe which have some high boiling points. The rest of the air along with N2 passes above the column is mixed with the vapors of He and Ne. After mixing they pass downward through two concentric pipes immersed in liquid O2. Again they are liquefied in the spherical bulb and collected in vessel C. The liquid in vessel C contains O2. He and Ne. A trace of O2 is also present in compartment C and is forced under pressure through B to G at the top of the apparatus. The liquid in A rich in O₂ is forced to the point F which lies much below column G. Now fractions entering column G are suddenly exposed to atmospheric pressure where it beings to boil and N2, He and Na escaped out. The O2 present in the liquid enters into column F and passes down in the liquid due to its less volatile property. This liquid O2 collected in vessel A consists of Ar, Kr, and Xe. It is evaporated by the heat of the incoming stream of cold compressed air which is taken out through the exit. This portion consists of O2, Ar, Kr, and Xe along with a minute quantity of N₂.
Separation of He and Ne:
The fractions containing N₂, He, and Ne are taken out from column C and passed through a spiral column placed in boiling N₂⋅N₂ of the mixture is liquefied in it while He and Ne escape out, A trace of N₂ is also present in the mixture of He and Ne are removed by passing over CaC2 and CaCl2 mixture as described earlier. The mixture contains a 3: 1 ratio of Ne and He. The mixture cooled at liquid H₂ temperature where Ne is converted into solid leaving He in the form of gas. The mixture of He and Ne is also separated by Dowar’s charcoal method.

Separation of Argon:

The second fraction that escaped from J contains liquid Cl2 along with Ar, Kr, Xe, and traces of N2was in compartment ‘ A: Compartment A contains a large: series of ‘B tubes‘ which are cooled in a bath of liquid N2 temperature. Nitrogen is not liquefied and is passed out. Ar mixed with residual O2 is passed down the column along with Kr and Xe. Ar and N₂ are more volatile and consist of a small amount of O2 that also rises above the column as shown in figure 4.23. This mixture contains 47%N2, 47%Ar, and 6%Cl2 in container ‘C‘ which has a series of tubes ‘D‘ kept at liquid N₂ temperature. They are not liquefied and passed out. Ar mixed with residual O2 passes down the column and is collected as a liquid at the bottom which becomes heated by compressed air and escapes through exit F along with O2. The O2 is removed by passing over red hot Cu and Ar is recovered in the pure state.

Fig. 4.23∗1 solation of argan from the liquid oxygen
Separation of Kr and Xe:

Kr and Xe are the least volatile and accumulated in liquid O2 at the bottom of column ‘ A ‘. The O2 is evaporated off by the heat of compressed air while Kr and Xe remained in a liquid state along with a trace of O2. This liquid flows through a rectifier column having a coil at the bottom through which compressed air is circulated which results in O2 being vaporized. The liquid mixture of Kr and Xe flows down at the base of the bottom of the apparatus used. Both can be separated by the treatment of another rectifier or by Dewar’s: method of separation.
Natural gas which is the main source of He can be obtained by a special method on a large scale. Natural gas consists of CH4, CO2, N2, and He as the main sources. CO2 is removed by absorbing It in conc.KOH solution. Pest gases are passed through a rectifying column to liquefy except N2. He and a little CH4. This mixture of gases is compressed at 100 atmospheric pressure and −200∘C. We get 99% of pure Hee leaving N2 and CH4 in a liquid state.

Physical properties of noble gases :

  1. The melting point and boiling point of inert gases are very low as given in table 4.19. This is due to the weak intermolecular forces or van der Waal forces of attraction. The melting point and boiling point increase with the increases of their atomic number.
  2. They are sparingly soluble in water. It also increases with the increase of their atomic number. All are colorless and odorless monoatomic gas. They do not combine among themselves or with other atoms. The spectroscopic studies of all these gases provide characteristic spectra on the basis that they can be identified. Helium gives D3 yellow lin o whereas neon gives orange-red spectral lines: The value of entropy of vapourization is very low due to weak van der Waal forces amongst them. It goes on decreasing from top to bottom in a group.
  3. They have large atomic radii, It also increases with the increase in their atomic number. The value of atomic radii is non-bonded radii. All the noble gases are diffusable through glasses, plastics, and rubbers. They are also diffusable through some metals.

Some Special properties of He :

Helium is unique amongst all the noble gases. It has the lowest value of melting point and boiling point. When extremely cooled all noble gases are converted into solid but helium exists in liquid form. It shows two different liquid phases which are known as He(i) and He(II).He(I) behaves as a normal liquid whereas He(II) is a superfluid which is an unusual state of matter. He(I) is liquid at 4.22 K but to great surprise, it boils vigorously and at 2.2∘C it stops boiling and is converted into He(II) whose thermal motion is stopped.

At λ point temperature He(l) is changed to He(II) with the abrupt change in physical properties. The thermal conductivity of He(II) is increased by 106 which is 800 times greater than for copper. It acts as a superconductor without any electrical resistance, It has a viscosity that is 1100th to that of gaseous H2. He(II) spreads to the whole surface at a temperature below the λ point. It flows up from the sides of the vessel and also over the edge to equalize the liquid level on both sides of the vessel.

Chemical Properties of Noble gases :

For a long time, inert gases are thought to be chemically inert. Before the year 1962, there was only evidence regarding the formation of molecular ions formed in a discharge tube and clathrate compounds. There are several molecular ions like He2+, HeH+, HeH2+and Ar2+   that are found to be formed in the discharge tube under high energy conditions. Their life is very short and can be detected by spectroscopic methods. They do not form natural covalent molecules.

Clathrate compounds:

Normally most of the elements form compounds of ionic or covalent nature. In the clathrate compound, atoms or molecules are held in the cavities of the crystal lattice of another compound. Since they are held in cavities hence they do not form any chemical bond. When an aqueous solution of 1,4 dihydroxy benzene (quinol) is crystallized at 10−40 atmospheric pressure in the presence of Ar, Kr, and Xe, a clathrate compound is formed.

After dissolving these clathrate hydrogen bond breaks down between quinol and noble gas trapped escapes away. Other small molecules like H2S, SO2, O2, CH2OH, and CH3CN also form clathrate just like Ar, Kr, and Xe. Due to the smallest size, He and Ne do not form the clathrate compounds. The composition of clathrate formed by quinol is 3:1 (3 quinols :1 inert gas). All the cavities of a clathrate do not trap inert gas. Water in the form of ice trap

Ar, Kr, and Xe form clathrate which is also known as “hydrolysis of noble gas” in which the ratio is 6H2O: 1 inert gas atom. Such compounds provide an easy means of storing radioactive isotopes of Kr and Xe produced in nuclear reactors.

Chemistry of Xenon :

First of all Bartlett and Lohman in 1962 oxidize O2 with the help of PtF6, a strong oxidizing compound. O2 is ionized to O2+ by absorbing the energy of 1165 kJ per mole which is nearly the same to oxidize Xe to Xe+(E=1170 kJ/mole). When the deep red vapor of PTF6 is mixed with an equal volume of Xe they produce a yellow solid at room temperature which was innocently thought to be xenon Hexa fluoro platinate (V); Xe+[PtF6]−. But in an actual sense, the compound is [XeF]+[Pt2 F11]−.

PtF6 + O2 ⟶ O2+ + [PtF6]− Xe[PtF6] + PtF6⟶25∘C [XeF] + [PtF6]−

+PtF5⟶60∘C[XeF]+ [Pt2F11]−

After some time it was also found that at 400∘C Xe reacts with F2 and gives a colorless solid XeF4. After these discoveries, there was rapid growth in the chemistry of inert gases, particularly Xenon. it is because the ionization energies of He, Ne, Arand Kr are much greater than the ionization energy of Xee. Krypton also forms KrF2 when reacts with F2. Xe reacts directly with F2 and forms xenon fluorides. These xenon fluorides form oxy compounds. Sometimes xenon also forms XeCl2 and XeCl4 compounds. Hence, there is the possibility of extensive chemistry of Xe which is as follows:
At 400∘C Xe directly reacts with F2 in a sealed Ni vessel to give different fluorides depending on the ratio of Xe and F2.
All these fluorides are white solids. At room temperature they get sublimed hence they can be stored in N vessel or Monel containers. Lower fluorides of Xe form higher fluorides when heated in presence of F2 at elevated temperature. All the fluorides are acting as fluorinating agents as well as strong oxidizing agents.

1- Reaction with H2: Xenon fluorides when react with F2 gas give Xenon and HF.

XeF2+H2⟶Xe+2HF

XeF4+2H2⟶Xe+4HF

XeF6+3H2⟶Xe+6HF

2- Xenon fluoride acts as an oxidizing agent and oxidizes Cl and I− to Cl2 and I2 respectively.

XeF2+2HCl⟶2HF+XO+Cl2

3- It oxidizes Ce(III) salts to Ce(IV) salts.

XeF4+SO42-+Ce2(SO4)3⟶2Ce(SO4)2+Xe+F2

4- It acts as a fluorinating agent

XeF4+2SF4⟶X0+2SF4XeF4+Pt⟶Xe6+PtF4XeF4+2C6H6⟶Xee+2C6H5 F+2HF.

5- XeF2 is soluble in water and undergoes slow hydrolysis.

2XeF2+2H2O⟶2Xe+4HF+O2

6- XeF4 reacts with H2O vigorously and gives XeO3.

3XeF4+6H2O⟶XeO3+12HF+3/2O2+2Xe

7- XeF6 also reacts violently with water and gives a highly explosive solid due to hydrolysis with moisture present.

XaF6+3H2O⟶XnO3+6HF

With a small quantity of H2O, it gives XeOF4. Silica also forms the same product.

XeF6+H2O⟶XeOF4+2HF2XeF6+SiO2⟶SiF4+2XeOF4

8- XeO3 is explosive white solid and hygroscopic and it also reacts with XeF8.

XeF6+2XeO3⟶3XeO2 F2
2XeF6+XeO3→3XOOF2
XeOF4+XeO3⟶2XeO2F2

9- Reaction with NaOH: It forms xenon ion with NaOH.

XeO3+NaOH ⟶ Na+[HXeO4]−

10- With conc.H2SO4 fluorides of XeO4 (Xenon tetroxide) which is explosive and volatile.

Complexes of Xenon fluorides:

All the pentavalent fluorides of the VA group and transition metals form complexes with XeF2.

XeF2 + MF5 ⟶ [XeF]+[MF5]–         M = P, As and Sb

XeF2 + 2MF5 ⟶ [XeF][M2F11]–     M = Nb, Ta, Ru, Rh, Os and Pt

2XeF2 + MF5 ⟶ [Xe5F3]+[MF6]–

In solid-state, the structure of some of the xenon flounder complexes is well known. The structure is intermediate between an ionic structure and a fully covalent bridge structure. The bridge structure is formed through F2.XeF4 forms only a few complexes with PF5, AsF5, and SbF5.XeF6 acts as a fluoride XeF6AsF5 : XeF8SnF5

With heavier alkali metal halides XeF6 act as a fluoride acceptor.
XeF6 + RbF ⟶ Rb + [XeF7]−
On heating [XeF7]−ion gets decomposed.

Uses of Inert gases :

  1. Argon is used to provide an inert gas atmosphere for metallurgical processes. It is used in the welding process of steel Ti, Mg, and At. In trace it is used in growing Si and Ge crystals for transistors, fluorescent lamps, radio, valves, and Griger-Muller radiation counters:
  2. Helium has the lowest b.p. amongst all noble gases hence it is used in cryoscopy to get the very low temperature required for superconductivity and lasers. It is used to cool down the gas in nuclear reactors. It is also used as flow gas in gas-liquid chromatography. It is also useful in weather tealloons and airships because of the inflammability of H2. One cubic matter of helium gas at atmospheric pressure is able to 1ft1Kg wt. Helium is also used to dilute O2 in the gas cylinders which is used by marine divers.
  3. Small amounts of neon are used in discharge tubes that glow with a reddish-orange color. Which is used for advertisements to give different colors.

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