1. The Noble Gases

2. Helium is less dense than air, so it’s great for all kinds of balloons. Helium balloons lift instruments into the upper atmosphere to measure atmospheric conditions.

3. The Noble Gases Even though hydrogen is lighter than helium, helium is preferred for these purposes because helium will not burn.

4. Uses for the Noble Gases The “neon” lights you see in advertising signs can contain any of the noble gases, not just neon. Electricity is passed through the glass tubes that make up the sign.

5. Uses for the Noble Gases The electricity causes the gas to glow. Each noble gas produces a unique color. Helium glows yellow, neon glows red-orange, and argon produces a bluish-violet color.

6. Uses for the Noble Gases Argon, the most abundant of the noble gases on Earth, was first found in 1894. Krypton is used with nitrogen in ordinary lightbulbs because these gases keep the glowing filament from burning out. Krypton lights are used to illuminate landing strips at airports, and xenon is used in strobe lights and was once used in photographic flash cubes.

7. Uses for the Noble Gases At the bottom of the group is radon, a radioactive gas produced naturally as uranium decays in rocks and soil. If radon seeps into a home, the gas can be harmful because it continues to emit radiation. When people breathe the gas over a period of time, it can cause lung cancer.

8. Why are they called the ‘noble gases’? They are all very unreactive. The noble gases all form colourless gases at room temperature. The name was changed to ‘noble gases’ as they were considered similar to the very unreactive precious metals gold and platinum, which are sometimes called ‘noble’ metals. Now only neon and helium have not yet been made to form compounds. Noble gases were originally called ‘inert gases’, as they were thought not to react with anything. Then in 1962, a British chemist, Neil Bartlett, made a compound with xenon.

9. Where are the noble gases? The elements in group 0, on the right of the periodic table, are called the noble gases. He Rn Xe Kr Ar Ne helium neon argon krypton xenon radon

10. Position of inert gases in the periodic table The inert gases were not known at the time of Mendeleef when he gave bis original classification of elements. He did not leave gaps for these gases although he left gaps for several other unknown elements. After their discovery a new group was introduced into the periodic table. In the modern periodic table, these may be placed in the zero group as well in the VIII group as well as in the VIII group because of the following reasons :

11. Reasons for placing in zero group: Chemical inertness: These elements are chemical inert. The existence of such a group is expected from the fact that there must a separating group between strongly electronegative halogens of VII A group and strongly electropositive alkali metals of IA group. Therefore, these elements should be placed in between VIIA group and IA group, i.e., in the zero group. Valency: Valency is defined as the combining capacity of the element. The electronic configuration of these elements given as: He: 2(1s), Ne: 2.8 (2s 2 p 6 ), Ar: 2,8,8 (3s 2 p 6 ), Kr: 2,8,18,8 (4s 2 p 6 ), Xe: 2,8,18,18,8 (5s 2 p 6 ), Rn:2,8,18,32,18,8 (6s 2 p 6 ) indicate that the atoms of these elements have saturated shells. Have, therefore no tendency either to lose, gain or share electrons with the atoms of other elements. In other words, their combining capacity or valency is zero. Therefore, these elements should be placed in the zero group.

12. Reasons for placing in VIII group: Electronic configuration: Electronic configurations of inert gases indicate that except helium which has 1s 2 configuration, all the elements have eight electrons in the outermost shell. Therefore, on this basis they should be placed in the VIII group. Analogy with other groups: In the modern periodic table, all the groups except VIII, are divided into two sub- groups A and B each. The elements of A sub-groups are normal elements, whereas those of B sub- groups are transitional. If the VIII group is also considered parallel to others, it should also be divided into two sub-groups viz. VIIIA and VIIIB. The VIIIA group should contain inert elements and VIIIB group should contain transitional elements. In such a case, the periodic table will be more symmetrical. Thus, from the above discussion it is clear that the inert gases may be placed in both the groups, i.e., zero and VIII.

13. However, for the general reference they are still considered as the members of the zero group. The position of these gases is further justified because of the facts namely: They are all colorless, odorless and tasteless gases, under ordinary conditions. The specific heat ratio, C p /C v is the same (1.66) for all gases, which indicates their mono-atomic character. They show a regular gradation in their physical and chemical properties. They do not have any resemblance with the elements of their neighboring groups. They are all chemically inert. More reasons for placing in zero group:

14. Excepting radon, all other noble gases occur throughout the universe in atomic state. Since radon is a radioactive element and hence decays rapidly, it does not occur in the free state. Other rare gases occur in the following sources: In the stars and intercellular space In the earth’s atmosphere In natural gas In minerals and spring waters. Occurrence of Noble Gases:

15. Reasons behind chemically inertness of rare gases A.They have no tendency either to lose, gain or share electrons with the atoms of other elements. Do not enter into chemical combination. B. No electrons are available for sharing with the electrons of other atoms. They cannot form covalent compounds. C. Electronegativity and electron affinity of these elements are zero. No tendency to attract or accept electrons and hence, are unable to form compounds. D. The ionization potentials of these elements are very high. Removal of electrons from the outermost shell is very difficult.

16. Facts indicating the mono atomic character of inert gases Specific heat ratio Value of physical constants Electronic configuration

17. History of discovery of rare gases Discovery of argon Discovery of helium Discovery of neon, krypton and xenon Discovery of radon

18. Isolation of rare gases from dry air (Chemical method) i. Isolation of inert gases mixture from the dry air: Rayleigh and ramsay first method Rayleigh and ramsay second method Fisher Ring’s method ii. Separations of the individual gases from the inert gases mixture

19. Isolation of Inert gases from liquid air (physical method) Separation of helium and neon from the 1 st fraction Separation of argon from the 2 nd fraction Separation of krypton and xenon from the 3 rd fraction

20. Uses of Noble Gases Of all the noble gases, only He and Ar which are available easily and plenty have got many uses. The uses are due to their chemical inertness and low boiling points. Important uses of noble gases are given in the following sections:

21. Uses of helium It is used in gas cooled atomic reactors as a heat transfer gas, i.e. as a cooling medium. This use of helium is because of the following properties of this gas: I)It has high thermal conductivity. II)It has low viscosity. III)It remains unaffected by irradiation. IV)It is inert in nature and hence does not undergo corrosion. Mixed with O 2, it is used in the treatment of asthma. Being light, this diffuses more rapidly than air through the partly choked lung passages. The mixture of He and O 2 is used for the artificial breathing of asthma patients. This mixture keeps the breathing steady. It is used for filling in balloons and airships as it is a light and non-combustible gas. For this purpose, a mixture of 85% helium and 15% hydrogen is generally used. The mixture of helium and oxygen is used for sea divers. The air proves harmful because nitrogen of the air dissolves in the blood at high pressure when the diver is under deep sea. When he comes to the surface, the pressure is suddenly decreased. The bubbles of nitrogen gas now escape from the blood and are collected at joints which cause pains. Helium taken in the place of air is much less soluble in blood at high pressure.

22. Uses of helium Liquid helium is used for producing low temperatures required for research work. For producing inert atmosphere in the welding and metallurgy of certain metals (e.g. Mg, Al, Ti etc.) which are easily oxidized. In the preservation of food. It is used for filling electrical transformers. It is used as a pressuring agent in rockets to expel liquid O 2 and liquid H 2. It is used as a shielding gas for arc welding as it protects both the arc and metal atmosphere. It is suitable for low temperature gas thermometry because this gas has low boiling point and has nearly ideal gas behavior. Helium nucleus (α-particles) is used as a bombarding particle for the artificial disintegration of atoms.

23. Uses of Neon It is used in neon discharge lamps and signs which are used for advertising purposes, since the neon lights are visible from long distances and even visible through fog and mist. Since the light of neon signs has a better penetrating power through fog and mist, the neon signs are also used in beacon lights for the safety navigation. Mixture of He and Ne does not conduct electric current until a certain limiting voltage is applied to it and therefore it starts conducting electric current. Based on this property, it is used in certain protective electrical devices likes voltage stabilizers, current rectifiers etc.

24. Uses of Argon i. A mixture of argon with 15% nitrogen is filled in electric bulbs to increase the life of the filament. ii. For producing inert atmosphere in welding and metallurgy of certain metals which are easily oxidized. iii. In filling fluorescent tubes and radio-valves. iv. It is mixed with neon in neon signs to obtain the lights of varying colors.

25. Uses of Krypton i. It is used in high efficiency miner’s cap lamps. ii. It is used in filling luminous tubes. iii. Krypton-85, one of the isotopes of Kr, is used in electric tubes for regulating the voltage and for testing the leaks. iv. Krypton-85 is also used for the measurement of the thickness of sheets of metals and plastics.

26. Uses of Xenon It is used in discharge tubes for producing high speed flash of bluish light used in quick photography. Liquid xenon is used in bubble chamber for the detection of gamma photon and neutral mesons. Uses of Radon i. It is used in the preparation of ointment for the treatment of cancer and other diseases. ii. It is used for photographing the interior of opaque materials for locating the defects in steel castings.

27. How many everyday uses of noble gases can you see below? What are the uses of noble gases?

28. Importance of Inert gases in theoretical Chemistry In the periodic table. In the elucidation of distribution of electrons in atoms. In the development of electronic theory of valency. In the discovery of isotopes. In radioactivity.

29. Properties of the noble gases The properties of the noble gases can be well explained by modern theories of atomic structure: “their outer shell of valence electrons is considered to be "full", giving them little tendency to participate in chemical reactions, and it has been possible to prepare only a few hundred noble gas compounds. “ The melting and boiling points for each noble gas are close together, differing by less than 10 °C (18 °F); that is, they are liquids over only a small temperature range.

30. Physical and atomic properties PropertyHeliumNeonArgonKryptonXenonRadon Density (g/mL)0.17860.90021.78183.7085.8519.97 Boiling point(K)4.427.387.4121.5166.6211.5 Melting point (K)0.9524.783.6115.8161.7202.2 Enthalpy of vaporization (kJ/mol) 0.081.746.529.0512.6518.1 Solubility in water at 20 °C (cm 3 /kg) 8.6110.533.659.4108.1230 Atomic number21018365486 Atomic radius (calculated) (pm) 31387188108120 Ionization energy (kJ/mol) 237220801520135111701037 Allen electronegativity4.164.793.242.972.582.60

31. Physical and atomic properties The noble gases have weak interatomic force and consequently have very low melting and boiling points. They are all monatomic gases under standard conditions, including the elements with larger atomic masses than many normally solid elements. Helium has several unique qualities when compared with other elements: its boiling and melting points are lower than those of any other known substance; it is the only element known to exhibit superfluidity; it is the only element that cannot be solidified by cooling under standard conditions—a pressure of 25 standard atmospheres (2,500 kPa; 370 psi) must be applied at a temperature of 0.95 K (−272.200 °C; −457.960 °F) to convert it to a solid.

32. Chemical properties Neon, like all noble gases, has a full valence shell. Noble gases have eight electrons in the outermost shell, except in the case of helium, which has two. They were once labeled group 0 in the periodic table because it was believed they had a valence of zero, meaning their atoms cannot combine with those of other elements to form compounds. However, it was later discovered some do indeed form compounds, causing this label to fall into disuse. Electron configuration, especially the outermost shells resulting in trends in chemical behavior: ZElement No. of electrons/shell 2helium2 10neon2, 8 18argon2, 8, 8 36krypton2, 8, 18, 8 54xenon2, 8, 18, 18, 8 86radon2, 8, 18, 32, 18, 8 The noble gases have full valence electron shells. Valence electrons are the outermost electrons of an atom and are normally the only electrons that participate in chemical bonding. helium 2 neon 2,8 argon 2,8,8

33. Compounds Structure of XeF 4, one of the first noble gas compounds to be discovered The noble gases show extremely low chemical reactivity; consequently, only a few hundred noble gas compounds have been formed. The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn. Xenon compounds are the most numerous of the noble gas compounds that have been formed. Most of them have the xenon atom in the oxidation state of +2, +4, +6, or +8 bonded to highly electronegative atoms such as fluorine or oxygen, as in xenon difluoride (XeF 2 ), xenon tetrafluoride (XeF 4 ), xenon hexafluoride (XeF 6 ), xenon tetroxide (XeO 4 ), and sodium perxenate (Na 4 XeO 6 ). Some of these compounds have found use in chemical synthesis as oxidizing agents; XeF 2, in particular, is commercially available and can be used as a fluorinating agent. As of 2007, about five hundred compounds of xenon bonded to other elements have been identified, including organoxenon compounds (containing xenon bonded to carbon), and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon itself. Compounds of xenon bound to boron, hydrogen, bromine, iodine, beryllium, sulphur, titanium, copper, and silver have also been observed but only at low temperatures in noble gas matrices, or in supersonic noble gas jets. In theory, radon is more reactive than xenon, and therefore should form chemical bonds more easily than xenon does. However, due to the high radioactivity and short half-life of radon isotopes, only a few fluorides and oxides of radon have been formed in practice.

34. Compounds An endohedral fullerene compound containing a noble gas atom. Noble gases can form endohedral fullerene compounds, in which the noble gas atom is trapped inside a fullerene molecule.

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