Chemistry of the Noble Gases, Chemistry tutorial

Introduction:

In the Modular year, you are familiar regarding the chemistry of a group of highly reactive elements, that is, the halogens. Such elements are helium, neon, argon, krypton, xenon and radon. These elements comprise Group 18 of the modern periodic table. If you compare Mendeleev's periodic table of the year 1871 by the modern periodic table, we will observe that it is remarkably identical in its coverage to the modern periodic table, by the exception that the Group VII (18) is missing. The elements of Group 18 were not identified at that time and have been discovered only about a 100 years back. As these elements encompass very low reactivity, they were termed as inert. Though, the word inert is no longer applicable to the group as a whole, as the heavier elements of this group form compounds and, therefore, are not inert. These elements have as well been termed as the rare gases, however as argon forms almost 1 % of the atmosphere, and the gases can be readily isolated via the fractional distillation of liquid air at low temperatures, this name is as well not very suitable. They are now termed as the noble gases by analogy by the noble metals, such as gold and platinum that are not very reactive. The exclusive chemical inertness of the noble gases is well reflected in the history of their discovery which was followed via a long gap of a few decades before xenon could be made to join by only the most electronegative elements, fluorine and oxygen.

Rediscovery of Noble gases:

The story of discovery and examination of one noble gases is one of the most brilliant and fascinating chapters in the history of science. Their discoveries can be traced back to year 1785, when Henry Cavendish investigated the composition of air. He mixed surplus oxygen by air and then passed electric sparks via the mixture. The oxides of nitrogen therefore formed, were eliminated by dissolving in alkali solution and the surplus of oxygen was eliminated by potassium sulphide. The residual gas, which was for all time left behind, was neither nitrogen nor oxygen. It didn't form more than 1/120th portion of the original volume of air. Time was not yet ripe for the discovery of noble gases. What Cavendish had actually isolated was, of course, a mixture of the noble gases; however he could not characterize them. It would be interesting for you to know that his figures about the volume of residual gas are remarkably close to the proportion of the noble gases in the atmosphere as we now know it. It was approximately a century after the investigation of the composition of air by Cavendish which advances in spectroscopy, periodic categorization and the study of radioactive elements made possible the discovery of all the six noble gases.

Among all the noble gases, first came the discovery of helium, which is unique in being the first element to be discovered extra-terrestrially before being found on the earth. In the year 1868 the French astronomer Pierre Janssen came to India to study the total eclipse of the sun. By utilizing a spectroscope he noticed a new yellow line close to the sodium D lines in the spectrum of the sun's chromospheres. This led two Englishmen, chemist E. Frankland and astronomer Sir J.  Norman Lockyer to recommend the existence of a new element, which, suitably, they named helium, from the Greek word 'Helios' meaning the sun. The terrestrial existence of helium was established via Sir William Ramsay in the year 1895. He illustrated that a gas present in trace amounts in the uranium mineral, cleveite, consists of a spectrum similar with that of helium. Five years later, he and Travers isolated helium from air. Cady and McFarland discovered helium in natural gas in the year 1905 when they were asked to examine a sample of natural gas which would not burn. 

Most of the progress in noble gas chemistry date from Lord Rayleigh's observations in the year 1894. In order to test Prout's assumption, that the atomic weights of all elements are multiples of that of hydrogen, Rayleigh made precise measurements of the densities of common gases and found, to is surprise that the density of nitrogen achieved from air by the elimination of O2, CO2 and H2O was consistently around 0.5% higher than that of nitrogen achieved chemically from ammonia. He noticed that a litre of nitrogen obtained from air weighed 1.2572 grams whereas a litre of nitrogen obtained from ammonia weighed just 1.2506 grams under the similar conditions. This small difference of around 0.0066 gram in a gram and a quarter made Rayleigh to suspect an undiscovered element in the atmosphere. This reflects not just the extraordinary experimental skill of Lord Rayleigh however as well his scientific and objective process of thinking and working that led to the discovery of a whole new group of elements.

Ramsay treated the atmospheric nitrogen repeatedly by heated magnesium and found that a small quantity of a much denser gas was left behind that would not join by any other element. Lord Rayleigh and Sir W. Ramsay found that the residual gas exhibited spectral lines that were not noticed earlier in the spectrum of any other element. In the year 1894, they announced the isolation of the noble gas that they named argon, from the Greek word 'argos' meaning idle or lazy, due to its inert nature. They as well realized that argon couldn't be put by any of the other elements in the groups already recognized in the periodic table.

In the year 1898, Sir William Ramsay and his assistant Morris W. Travers isolated neon (from the Greek term meaning new) through the fractional distillation of impure liquid oxygen. Shortly afterward, they exhibited that the less volatile fractions of liquid air have two other new elements, krypton (from the Greek term meaning hidden) and xenon (from the Greek term meaning stranger).

Element 86, the last member of the group is a short lived radioactive element. This was isolated and studied in the year 1902 by Rutherford and Soddy and has been named as radon as it is made up by radioactive decay of radium.

Position of Noble Gases in the Periodic Table:

Due to their approximately inert chemical nature, the noble-gases occupy a peculiar place in the chemistry. Mendeleev had not left any vacant spaces for the noble gases in his periodic table however he had left such spaces for some other elements that were not known at that time. The reason was that he could not visualize the existence of a whole group of elements devoid of all chemical reactivity in ordinary conditions. Thus, the discovery of the noble gases at the outset seemed to upset Mendeleev's plan of categorization of elements. 

After studying the chemical nature of the noble gases, Ramsay proposed a new group in the Mendeleev's periodic table to put up these elements. He put this group after the halogens and before the alkali metals in the periodic table. As we are familiar, in the long form of the periodic table, the noble gases occupy the last column of the table. The inclusion of the noble gases has actually enhanced the periodic table as it gives a bridge between the strongly electronegative halogens and the strongly electropositive alkali metals.

Primarily the group comprising of noble gases used to be known as the Group zero or the Group VIII A. However according to the latest IUPAC convention, number 18 has been assigned to this group. Though, the position of the group in the periodic table remains unchanged, that is, after the halogens at the end of each and every period.

Occurrence, Isolation and uses of noble gases:

The noble gases comprise about 1.18% by volume of the dry air at sea level. Of all the noble gases, argon is the richest comprising 0.93% by volume of the dry air. As illustrated in the table shown below, He, Ne Ar and Rn are as well found occluded, although in very minute quantities, in igneous rocks. Some natural spring waters contain small amounts of dissolved He, Ne and Ar. Big reserves of helium have been lately discovered in hot water springs of Bakreswar and Tantloi in West Bengal. The gas coming out of such springs contains around 1.8% of helium. Natural gas in some parts of the world, specifically in U.S.A., contains as high as 7% of helium. 

The major source of Ne, Ar, Kr and Xe is air. Due to the difference in their boiling points (Table shown below), these gases are separated via fractional distillation of liquid air. However, the concentration of helium in the air is five times that of Kr and 60-times that of Xe, recovery of He from this source is uneconomical. The major source of helium is natural gas which comprises predominantly of hydrocarbons and nitrogen. These are liquefied via cooling under pressure. The residual helium is purified via passing it over activated charcoal cooled by liquid air. The charcoal absorbs traces of heavier noble gases, leaving pure helium. Radon is achieved by allowing radium or any of its salts to decay for several weeks in a sealed vessel.

Helium, being extremely light and non-inflammable is employed to lift weather balloons and to inflate the tyres of large aircrafts, thus increasing their payload. A mixture of 80% He and 20% O2 is utilized in place of air for breathing via deep-sea divers. As He is much less soluble in blood than N2, it doesn't cause sickness via bubbling out when the pressure is discharged as the diver comes to the surface. The boiling point of helium is the lowest of any known substance. Therefore, it is extensively employed in cryoscopy as a cryogen. You should have heard of superconductivity that is expected to bring revolutionary changes in our life. Thus far, helium gives the only practical means of studying and using such low temperature phenomena as superconductivity, although intensive research is going on and claims have been made up of achieving superconductivity in several materials at 125 K. Again, heat produced in the high temperature reactor (HTR) should be extracted by means of an appropriate coolant. Helium serves as an outstanding coolant in such reactors. It is as well employed as a flow gas in gas liquid chromatography and in microanalysis.

Helium and argon are employed to give an inert atmosphere in several chemical reactions, in welding operations of Mg, Al, Ti and stainless steel and in zone-refining of silicon and germanium. The Argon is extensively utilized in place of nitrogen in incandescent electric bulbs and radio tubes to prevent the oxidation and evaporation of the metal filament. Neon, argon, krypton and xenon are employed in discharge tubes - the so called neon lights for advertising, the colour generated based on the particular mixture of gases utilized. Radon finds out a limited use in the cancer treatment.

Table: Composition of dry air

Gas      % by volume     B.P. (K)

N2           78.03              77.2

O2           20.99              90.1

Ar           0.93                87.2

CO2         0.033             194.7

Ne           0.0018           27.2

H2           0.0010            20.2

He           0.0005           4.2

Kr           0.0001           119.6

Xe          0.000008        165.1

Superconductivity is the phenomenon in which the material offers no resistance to the flow of electricity. This would, thus, allow transmission of electrical energy with practically no loss.

General Characteristics:

The entire noble gas elements are colourless, odorless and tasteless monoatomic gases. Certainly, they are the only elements which exist as uncombined gaseous atoms at room temperature and one atmosphere pressure. Each and every atom, behaves as if it is efficiently isolated. Some of the properties of noble gases are summarized in the table shown below:

Table: Properties of Noble gases

1762_Properties of Noble gases.jpg

You can observe from the table that all the noble gases contain eight electrons in their valence shell apart from helium which consists of only two electrons. Till the year 1962, the noble gases were considered to be inert as their compounds were not known. Lewis and Kossel in preparing their electronic theory of Valence in the year 1916 stipulated that a grouping of eight electrons or an octet in the valence shell symbolizes a very stable configuration. Therefore, they introduced the octet rule. According to this, the reactions of elements can be illustrated in terms of their tendency to accomplish stable electronic configuration of the nearest noble gas, ns2np6, by gaining, losing or sharing of electrons.

As all the noble gases encompass the stable ls2or ns2-np6 configuration, they encompass the highest ionization energies as compared to other elements in their periods. This reflects their reluctant for the chemical reactivity. Analogously, the electron affinity of such elements is either zero or consists of a small positive value. Thus, they are not capable to accept electrons to form unions. As we go down the group, the ionization energy of the noble gases reduces. Therefore, there is an increase in the chemical reactivity of the noble gases as we go down the group from helium to radon.

As, there are no usual electron pair interactions between the noble gas atoms, the only interactions are weak van der Waals forces. Thus, they encompass very low boiling and melting points in comparison with those of other elements of the comparable atomic or molecular weights. However, boiling and melting points of helium are the lowest of any known substance. The van der Waals attraction between the molecules atoms rises with the increase in the number of electrons per molecule or atom; heavy molecules having more electrons attract one other more strongly than the lighter molecules. Therefore, the van der Waals forces between the noble gas atoms rise as we move down the group from helium to xenon. As a result, melting and boiling points rises with the increase in the atomic number.

Helium consists of two isotopes, 1He and 4He. The latter comprises almost 100% of the atmospheric helium. Whereas 3He behaves normally, 4He consists of strange properties. Whenever cooled below 2.2 K at one atmosphere pressure, ordinary liquid 4He, known as helium-I changes to an abnormal form known as helium-II. The temperature at which this transition of He-I to He-II occurs is termed as Lambda point.

Beneath this temperature, its thermal conductivity raises a million fold and the viscosity becomes efficiently zero, therefore it is illustrated as a super-fluid.

All the noble gases, particularly helium, encompass tremendous capability to diffuse via almost all kinds of glass, rubber, PVC and so on.

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