GROUP 18 NOBLE GASES OR INERT GASES
Group 18 of the periodic table consists of helium, neon, argon, krypton, xenon and radon. All these are gases under ordinary conditions of temperature and pressure. All of them (except Rn) are present in air in traces. Rn is obtained from radio active distintegration of radium.
On account of their very minute quantities in atmosphere, they were named as rare gases. Due to their chemical inactivity these were named as inert gases. A number of xenon compounds and two krypton fluorides were prepared and thus they were named as noble gases.
All these elements possess ns2 np6 configuration. The differentiating electron enters into p-sub shell and thus are included in p-block elements.
Table Electronic Configuration of Group 18 elements
Atomic Number : 2 Electronic Configuration : 1s2 Group Number : 18 Periodic Number : 1
Atomic Number : 10 Electronic Configuration : 1s22s22p6 Group Number : 18 Periodic Number : 2
Atomic Number : 18 Electronic Configuration : 1s22s22p63s2 Group Number : 18 Periodic Number : 3
Atomic Number : 36 Electronic Configuration : 1s22s22p63s23p63d104s2 4p6 Group Number : 18 Periodic Number : 4
Atomic Number : 54 Electronic Configuration : 1s22s22p63s23p63d104s2 4p64d105s2 5p6 Group Number : 18 Periodic Number : 5
Atomic Number : 86 Electronic Configuration : 1s22s22p63s23p63d104s2 4p6 4d104f145s2 5p65d106s2 6p6 Group Number : 18 Periodic Number : 6
ISOLATION OF NOBLE GASES
The noble gases are isolated from air by removing oxygen and nitrogen from air free from carbon-di-oxide, water vapour, dust particles, etc., This can be accomplished by either chemical methods or physical methods. In the chemical method, the unwanted gases are removed by means of compound formation while in the physical method, these are removed by the fractional evaporation of liquid air.
The first step in this method is to isolate the noble gases mixed together, from the atmosphere by passing repeated electric sparks in air so as to remove nitrogen and oxygen as nitrogen dioxide (N2 + 2O2 ® 2NO2 ). The second step
is to separate the various constituents from one another taking advantage of the fact that they can be adsorbed on activated charcoal at different temperatures.
Step 1 Removal of oxygen and nitrogen of the atmosphere as Nitrogen
Ramsay - Raleigh's method:- A mixture of air and oxygen is constantly admitted into a glass globe of about 50 litres capacity. Two platinum electrodes are introduced and a discharge from a transformer of about 6000 - 8000 volts is passed by the action of which nitrogen and oxygen rapidly combine to form oxides of nitrogen. The oxides are dissolved out in a solution of sodium hydroxide continuously circulated through the flask.
N2+ O2 ® 2 NO
2 NO + O2 ® 2NO2
2NO2 + 2NaOH ® NaNO3 + NaNO2 + H2O
Oxygen if any is removed by introducing alkaline pyrogallol in the globe.
The supply of air and electric discharge is shut after some time and the remaining mixture of noble gases is pumped out.
Step 2 Separation of noble gases (DEWAR'S METHOD)
The mixture of noble gases obtained by the above method is separated into individual constituents by the use of coconut charcoal which adsorbs different gases at different temperatures.
The mixture of noble gases is passed into a double-walled bulb containing coconut charcoal and placed in a low temperature bath at 173K. It is allowed to remain in contact with the charcoal for about half an hour. At 173K, only argon,
krypton and xenon are adsorbed by the charcoal while helium and neon remain unadsorbed. These are pumped out and collected.
The mixture of helium and neon is kept in contact with coconut charcoal at 93K which completely adsorbs neon leaving free helium.
The charcoal at 173K containing argon, krypton and xenon is placed in contact with another charcoal at the temperature of the liquid air when argon diffuse into the other charcoal.
The temperature of the first charcoal (temp.173K) still containing krypton and xenon is raised to 183K when krypton is set free while xenon remain adsorbed in the charcoal. When it is heated, xenon is recovered.
XENON FLUORIDE COMPOUNDS
Xenon forms three binary Fluorides XeF2, XeF4, and XeF6 by the direct union of elements under appropriate experimental conditions.
Xe+F2 --- 673K -- > XeF2
Xe + 2 F2 --- 673K -- > XeF4
Xe+ 3 F2 --- 573K -- > XeF6
PROPERTIES: Xe F2, Xe F4 and Xe F6 are colourless crystalline solids subliming readily at 298K. They are powerful fluorinating agents. They are readily hydrolysed by even traces of water. For example.
2Xe F2 + 2 H2O ® 2Xe + 4HF + O2
Structure: The structure of the three xenon fluorides can be deduced from VSEPR theory. XeF2 and XeF4 have the linear and square planar structure respectively. XeF6, has 7 electron pairs (6 bonding and one lone pair) and thus
have a distorted octahedral structure in the gas phase.
USES OF NOBLE GAS
1. Because of its lightness and non-inflammability helium is used to filling balloons for meteorological observations.
2. Because of its lightness it is used in inflating aeroplane tyres.
3. Helium oxygen mixture is used by deep-sea divers in preference to nitrogen oxygen mixtures. It is much less soluble in blood than N2. This prevents "bends" which is the pain caused by formation of nitrogen bubbles in blood veins when a diver comes to the surface.
4. A mixture of oxygen and helium is used in the treatment of asthma.
5. Liquid helium (b.pt 4.2K) is used as cryogenic agent for carrying out various experiments at low temperatures.
6. It is used to produce and sustain powerful super conducting magnets which form essential part of modern NMR Spectrometers and Magnetic Resonance Imaging system (MRI) for clinical diagnosis.
1. Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes.
2. Mixed with helium it is used to protect electrical instruments from high Voltages.
3. It is also used in beacon lights for safety of air navigation as the light possesses fog and storm-penetrating power.
4. Neon light is used in botanical gardens as it stimulates growth and helps the formation of chlorophyll.
1. Mixed with 26% percent nitrogen it is used in gas filled electric lamps. 2. It is also used in radio valves and tubes.
(D) KRYPTON AND XENON
1. Krypton and xenon are also used in filling incandescent metal filament electric
2. They are also used to a small extent in discharge tubes.
1. It is used in radioactive research and in radiotherapy for treatment of cancer.