11. GROUP 18 ELEMENTS
(ZERO GROUP ELEMENTS)
Synopsis :
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Noble gases belong to ‘O’ group and P - block.
Helium, Neon, Argon, Krypton, Xenon and Radon are collectively known as noble gases or
Aerogens or rare gases.
• They have completed s and p orbitals in the outer shell.
• As they are chemically inert these are known as inert gases.
• Since some compounds of these elements are prepared recently they are rightly called as noble
gases instead of inert gases.
• As they are rarely present in nature they are called rare gases.
• These elements are also known as aerogens because they are available in air
• These elements are placed in between the most electronegative halogens and the most
electropositive alkali metals.
• These elements Show 'O' oxidation state.
• Every period starts with + 1 oxidation state and ends with - 1 oxidation state. The presence of
noble gases with 'O' oxidation state in between them is justified.
• All these elements have the general electronic configuration ns2 np6 in their valence shell
except helium, which has1s2 configuration.
• He has no penultimate shell
Ne has 2 electrons in penultimate shell
Ar has 8 electrons in penultimate shell
Kr, Xe, Rn have 18 electrons in penultimate shell
The only element which has 32 electrons in the anti penultimate shell is Radon.
Discovery of noble gases:
• Helium was discovered in the chromosphere of sun during the total solar ecclipse by Janssen
and Lockyer. D3 line in the spectrum is related to Helium. Helios means sun.
• Later this gas was separated from nitrogen by Ramsay.
• Raleigh discovered argon in atmospheric nitrogen. He first noticed nitrogen obtained from air is
heavier than pure nitrogen and it may be due to the presence of some heavier gas.
• Argon means lazy
• Later it was found that the argon obtained from air is a mixture of several inert gases.
• Ramsay and Travers isolated 16 litres of liquid argon from the atmospheric nitrogen and
subjected to fractional distillation at different reduced pressures and separated Neon. Neon
means new.
• Ramsay evaporated one litre of liquid air untill a very small amount is left over.
• The spectrum of the small amount of liquid air indicated the presence of a new element and
named it as krypton. Krypton means hidden.
• Ramsay separated yet another element from krypton by fractional distillation and named it as
xenon. Xenon means stranger.
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Group 18th Elements(Zero group elements)
• Radon was discovered from the radioactive disintegration of radium.
Occurrence:
• Except radon all the noble gases occur in free state in atmospheric air, sun, stars, natural gas
etc.
• Helium is present in natural gas to an extent of 2%.
• It is found in minerals in occluded form.
• % by volume in air: Ar > Ne > Kr > He > Xe
• % by weight in air: Ar > Ne > Kr > Xe > He
Separation of noble gases from air:
• Noble gases are separated by the following two methods.
1. Physico - Chemical methods
2. Physical methods
Physico - chemical methods:
Chemical methods involve separation of noble gases from air and Physical method involves
separation of noble gases from one another.
Ramsay - Rayleigh's first method:
• CO2 is removed from air by passing through soda lime and potassium hydroxide solution.
• O2 is removed by passing air through finely divided copper in a long tube.
• N2 is removed by passing air over hot Mg ribbon.
• The process is repeated till oxygen and nitrogen are completely removed.
Ramsay - Rayleigh's second method:
• A 50 litre round bottomed flask is fitted with a five holed rubber cork.
• Platinum electrodes are introduced into two holes.
• NaOH solution is pumped into the flask through one hole and it emerges out of the flask
through another hole.
• A mixture of dry air and oxygen mixed in the ratio 9:11 is passed into the flask through the
other hole.
• Electricity is passed through the electrodes at a potential difference of 6000 to 8000 volts.
• N2 and O2 present in air combine to form the oxides of nitrogen, which are absorbed by NaOH
solution.
• The mixture of noble gases contains a little oxygen which is removed by passing through
alkaline pyrogallol solution.
N2 + O2→2NO
2NO + O2 →2NO2
2NO2+2NaOH →NaNO2 + NaNO3+H2O
Fischer - Ringe's method:
Air is passed through a mixture of 90% CaC2 and 10% CaCl2 heated to 800°C in an iron tube.
CaC2 + N2 → CaCN2 + C
C + O2 → CO2
2C + O2 → 2CO
N2 and O2 are thus removed from air.
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Group 18th Elements(Zero group elements)
CO is converted into CO2 by passing over hot CuO and CO2 is absorbed by KOH solution.
CO + CuO → Cu + CO2
2KOH + CO2 → K2CO3 + H2O
Separation of noble gases from one another by Dewar's method (physical method)
• This method is based on the adsorption by activated charcoal.
• All the noble gases except helium can be adsorbed over activated charcoal.
• Adsorption of inert gas on the charcoal depends on temperature.
• Adsorption of inert gases on coconut charcoal increases with the increase of the atomic weight.
• Inert gas with low atomic weight is more adsorbed at low temperature.
• The mixture of inert gases is brought in contact with coconut charcoal kept in Dewar's flask at
173K.
• Ar, Kr and Xe are adsorbed and He and Ne remain unadsorbed at 173K.
• The unadsorbed He and Ne are seperated.
• The mixture of He and Ne is introduced into another bulb containing coconut charcoal at 93K
• At this temperature only Ne is adsorbed leaving behind helium which is seperated.
• Ne is recovered by warming the charcoal.
• The first charcoal having adsorbed Ar, Kr and Xe is brought in contact with another charcoal at
77 K.
• Ar, being a lighter gas diffuses into this charcoal at 77 K and is recovered by warming
separately.
• The temperature of the first charcoal at 77 K and is recovered by warming separately.
• The temperature of the first charcoal containing Kr and Xe is raised to 183 K.
• At this temperature Kr comes out of the charcoal.
• Xe which is in the adsorbed state is recovered by warming the charcoal.
Schematic representation of separation of noble gases: Noble gas mixture - He, Ne, Ar, Kr, Xe
Kept in contact with coconut charcoal at 173 K
gases adsorbed
Ar, Kr, Xe
unadsorbed gases
He, Ne
In contact with charcoal
at liquid air temp (77 k)
Adsorbed
gas Ar
In contact with
charcoal at 93 k
Gases left in the
first charcoal
Kr, Xe
unadsorbed adsorbed
gas Ne
gas He
Temperature raised to 183 K
Gas evolved Kr
Xe is in the adsorbed
form state Xe
warmin
Xe is evolved
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Group 18th Elements(Zero group elements)
Separation of noble gases from liquid air:
• Based on the difference in boiling points, noble gases can be separated from liquid air and from
one another by fractional evaporation method.
• B.P. order: He < Ne < N2 < Ar < O2 < Kr < Xe
• The process is done in Claude's apparatus.
• Liquid air on fractional distillation gives two fractions. The entire process is schematically
represented below.
Isolation of Argon, Krypton and Xenon :
Liquid air 77K
Fractional distillation
Liquid nitrogen containing
h li
Nitrogen is
removed as
CaCN2
Ne is
liquefied
d
Liquid oxygen containing
argon, krypton and xenon.
Passed over heated CaC2
Cooled in liquid
Mixture of helium
and neon gases
Cooled in liquid
H2 (20 K)
nitrogen
Ar as gas+ small
amount of oxygen
Kr + Xe +
oxygen
evaporation
He remains
as gas
Oxygen as
Fractional
distillation
Passed over
heated Cu to
Kr
Xe
Ar
Physical properties:
• These are colourless, odourless and tasteless gases.
• All these are monoatomic gases.
• Density melting point boiling points atomic weight and atomic radius increase down the group
with increase in atomic number.
• There are weak Vanderwaal's forces of attraction between the atoms.
• Vanderwaal's forces increase with the increase in size of the atoms.
• These elements have the highest ionization potentials in the periodic table due to stable
configuration.
• He has the highest ionization potential in the periodic table.
• Ionization potential decreases with the increase in atomic size down the group.
• These elements have 'zero' electron affinity due to the completely filled S and P orbitals.
• Heat of vaporisation increases from He to Rn.
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Group 18th Elements(Zero group elements)
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Solubility in water increases from He to Rn.
Least soluble and least readily liquifiable gas among all the known gases is He.
The ease of liquification increases from He to Rn with the increase in Vanderwaal's forces of
attraction.
• At 4.2K He can be condensed to a liquid called helium - 1.
• When it is cooled to 2.2 K it gives another liquid Helium-ll which possesses the properties of a
gas.
• Helium-ll shows unusual properties and has extremely low viscosity and very high thermal
conductivity.
• It has been termed as a degenerate gas or a fourth state of matter of a superfluid and it flows
upwards.
• Xenon hexafluoro platinate XePtF6 was the first noble gas compound reported by N. Bartlett.
Reactivity:
• He and Ne are chemically inert and they do not form any compounds. Their chemical inertness
is due to very high ionization potential, zero electron affinity and the absence of vacant dorbitals in valence shell.
• Ar, Kr and Xe will show some reactivity due to low ionization potentials and presence of
vacant d-orbitals in valence shell.
• Xe is more reactive than Ar and Kr due to it's low ionisation potential.
• Radon is radioactive and it will not show chemical reactivity.
• Xe shows tendency to lose electrons in many of it's reactions. Therefore, Xe combines with
only more electronegative elements like F and O' or electronegative groups like OSeF5 and
OTeF5.
• Xe does not combine with less electronegative elements like
F
Cl2or N2.
Compounds of xenon:
Xe
• Xenon combines with flourine to form XeF2, XeF4, XeF6 under
various conditions.
Structure of XeF2:
F
• In the formation of XeF2, xenon atom undergoes
3
sp d hybridization in it's first excited state.
• The hybrid orbitals are pointed towards the corners of a trigonal bipyramid.
• Three orbitals occupy the equitorial positions and the remaining two orbitals occupy the axial
positions.
• The equitorial orbitals have lone pairs of electrons and the axial orbitals have shared pairs of
electrons.
• XeF2 molecule is linear and the bond angle is 180°.
Structure of XeF4:
• The hybridization of Xe is sp3d2. Xe is hybridized in it's 2nd excited
state.
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F
F
F
Xe
F
Group 18th Elements(Zero group elements)
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The molecule has square planar structure.
Xe contains 4 bond pairs and 2 lone pairs. The 4 bonding orbitals are at equatorial positions and
2 nonbonding orbitals are at axial positions.
Structure of XeF6:
• The hybridization of Xe is sp3d2. Xe is hybridized in it's 3rd excited state.
F
• Xe contains 6 bond pairs and 1 lone pair. Out of these 7 sp3d3 orbitals, five
are at equatorial plane and two are at axial positions. Lone pair occupies
one of the axial positions.
• Due to the presence of a lone pair of electrons it possesses distorted octahedral shape.
F
F
Xe
F
Oxides of xenon:
• Xenon combines with oxygen to form XeO3 and XeO4.
F
• In XeO3, xenon is in + 6 oxidation state. It is hygroscopic and explosive.
Structure:
• Xe atom undergoes sp3 hybridization
• The three 'p' electrons are excited to vacant d - orbitals.
• Xe has 3 sigma bond pairs and 1 lone pair. Therefore shape of XeO3 is
Xe
pyramidal and bond angle is decreased from 109°.281 to 103°.
• In XeO3, there are 3 σ bonds and 3π bonds.
• All the 3σ bonds involve sp3 - p overlapping.
• All the 3π bonds involve d - p overlapping.
• The bond angle decreases from 109°281 to 103o due to the lone pair bond pair repulsions.
• Xe is linked to each oxygen atom by a double bond.
Xenon tetraoxide XeO4:
• It is formed when sodium or barium perxenate is treated with conc. H2SO4 at room
temperature.
Na4XeO6 + 2H2SO4 →XeO4+2Na2SO4 + 2H2O
Ba2XeO6 + 2H2SO4 → XeO4 +2BaSO4 + 2H2O
• It is an unstable compound and readily dissociates into Xe and O2.
XeO4 →Xe+2O2
Structure:
• The hybridixation of Xe is sp3. The three 'P' electrons and one 'S'
electron are excited to vacant d - orbitals.
• It has regular tetrahedral shape and bond angle is 109°.
• Each oxygen atom is linked to Xe atom by a double bond.
• The molecule contains 4σ bonds and 4π bonds and there is no lone
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Xe
Group 18th Elements(Zero group elements)
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pair.
All the 4σ bonds involve sp3 - p overlapping
All the 4π bonds involve d - p overlapping
Uses of noble gases:
Helium:
• As helium is a light and non combustible gas it is used in filling the meteorological balloons.
• A mixture of 20% oxygen and 80% helium is used for artificial respiration by deep sea divers.
This mixture is used by asthma patients for respiration.
• Liquid He is used as a cryogenic liquid for producing low temperatures.
• He gas is used in gas thermometers.
• It is used as heat transfer agent in nuclear reactors.
• It is used in electric transformers.
• It is used in welding of Mg, Al and stainless steel.
• It is used to provide inert atmosphere in the preparation of Mg, Al etc.
Neon:
• It gives orange red glow in a discharge tube at 2 mm of pressure.
• It is extensively used in glow lamps known as neon tubes for advertising purposes.
• When Ne is mixed with Hg vapor or Ar it produces various colors.
• The glow of neon lamps is visible even in fog and mist hence it is used in signal lights.
• It is used in safety devices, relays and rectifiers as it has a capacity for carrying high voltage
currents.
Argon:
• A mixture of Ar and mercury vapour is used in fluorescent tubes.
• It is used in filling electric bulbs.
• It is used in filling electric counter tubes, thermoionic tubes and other discharge tubes.
Krypton:
• It is used in miners cap - lamps.
• The isotope Kr-85 is used in measuring the thickness of plastic sheets, metal sheets, joints and
in electronic tubes for voltage regulations.
Xeon:
• It is used in photographic flash bulbs.
• Liquid xenon is used in detecting neutral mesons and gamma photons in the bubble chamber.
Radon:
• Radon is used in the treatment of cancer.
Radon is used in detecting the defects in steel castings.
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