UNIT 9 ELElMENTS OF GROUP 16 Structure 9.1 Introdbction objectives .9.2 Occurrence, Extraction and Uses OccutraKz Extraction uses 9.3 General Characteristics Physical Properties Chemical Properties 9.4 Compounds of Group 16 Elements Hydrides Hahdes Oxwles 9.5 Oxoacids of Sulphur Sulphurous Acid Sulphunc Acid Peroxoacids of Sulphur 7hiosulphuric Acid 9.6 Anomalous Behaviour of Oxygen 9.7 Summary 9.8 Terminal Questions 4.9 Answers 9.1 INTRODUCTION - , In the previous unit, you studied the Group 15 elements. In this unit, we shall discuss Group 16 elements, viz., oxygen, sulphur, selenium, tellurium and polonium. These elements are called'chalcogens or the ore forming elements. This name was derived from the Greek npme for copper, since most of the copper ores contain members of this group. Compared to Groups 14 and 15 elements, the elements of this group are even more non- metallic in their behaviour. These also exhibit a gradual change towards metallic ;haracteristics down the group. Thus polonium, the last member of the group, is almost metallic. We will deskribe the occurrence and extraction of these elements and also their allotropic modifications. Uses of these elements and some of their compounds will also be described. We shall then go on to discuss the-general behaviour of these elements and their important compounds like hydrides. halides, oxides and oxoacids. In the next unit, you will study Group 17, which consists of halogens. After studying this unit, you should be able to : describe the occurrence, extraction and uses of the elements of Group 16, describe the allotropic forms of all these elements, discuss the general characteristics of Group 16 elements, describe the geqeral properties of hydrides, halides and oxides of Group 16 elements, and list different oxoacids of Group 16 elements with an emphasis on the oxoacids of 9.2 OCCURRENCE, EXTRACTION AND USES Except oxygen and sulphur, other elements of the groupare relatively scarce. While oxygen sustains all life on this planet, other elements and their compounds also find a variety of uses. We shall discuss the occurtFnce, extraction and uses of these elements in this section. 9.2.1 Occurrence Oxygen is the most abundhnt of all elements on earth. Dry air contains 20.946% oxygen by volume in the free form. Oxygen forms about 46.6% by weight df the earth's crust including the oceans and the atmosphere. Most of this combined oxygen is in the form of silicates. oxides and water. Water is one of the most abundant compounds on earth, present not only in oceans which cover three fourths of the earth's surface but also in the atmosphere as vapour and in the subterranean aquifers. Living organisms also contain large amounts of water. Water contains 88.8% of oxygen by weight. By contrast, the abundance of sulphur in the earth's crust is only 0.03-0.19. It is often found as the free element near volcanic regions in lapan. Sicily, Mexico and Luisiana in USA. Combined sulphur exists primarily in sulphates and sulphides. such as gypsum. CaSO,. 2H,O. barite, BaSO, and epsom salt. MgSO,. 7H,O. The sulphides include~galena. PbS. zinc blende. ZnS. chalcopyrite CuFeS,. iron pyrite. FeS, and cinnabar. HgS. etc It also occurs in mineral springs as H?S. Along with C. H. 3. N and P, sulphur is one of the twenty-three odd elements essential for life. It is a constituent of substances such as eggs, wool, hair and also mustard, garlic, cabbage, etc. Selenium and tellurium also occur as sulphide ores. Polonium occurs naturally as a decay product in thorium and uranium minerals. Allotropic forms All the elements of the group show allotropy. Oxygen exists in two allotropic forms. Dioxygen. 02,is a diatomic gas, paramagnetic in nature. Lewis structure of oxygen molecule with a pair of covalent bonds between two oxygen atoms, as shown in the margin is inadequate in explaining its paramagnetic nature (Fig.9. I). This structure with all paired electrons is expected to be diamagnetic rather than paramagnetic. Paramagnetism of oxygen can be explained on the basis of Molecular Orbital Theory. This theory has been explained in Selenium has been named after the goddess of moon 'Selene' detail in Unit 5 of the 'Atoms and Molecules' course. The molecular orbital configuration of 0, molecule can be represented as : Polonium was named after Marie Curie's home counlry. Poland. She ,124. rr* 2p ',. was also the discoverer of this KK, a 2s'. a* 2s2,a $7; , elernenl. ~2~:' rr* 2p! The presence of two unpaired electrons in the antibonding orbitals explaiqs the experimentally observed paramagnetic behaviour. Ozone, 03.the other allotropic form of oxygen is a triatomic. pale blue gas. The only Fig 9.1 : Lewis struclure ol' O2 method used to make ozone commercially is to pass gaseous oxygen or air though a high molecule. voltage electric discharge called a silent electric dkcharge. Perhaps you know that the earth is covered by a layer of ozone which protects us from injurious ultraviolet rays coming from the sun. In the upper atmosphere at altitudes ranging from about 15-24 km. ozone is formed in appreciable amounts from oxygen by absorption of ultraviolet radiation from the sun. This radiation first splits O2 molecule into oxygen atoms, which react with O2 molecules to give 03. Ozone alko absorbs ultraviolet light. This causes the 0, to decompose and form 0, again. 20, 30, The absorption of uv radiation by O3 servesa twofold purpose. It protects the inhabitants of our planet from injurious radiation and mliintains an equilibrium between the concenwations of 0, and 0,. Recently there has been a serious concern about the depletion of this layer. Nitric oxide froni emissions of supersonic jets and chlorotluorocarbons used as aerosol propellants and as refrigerants have been identified as the main culprits. There is considerable. international effort to save the protective ozone layer. Sulphur displays allotropy to a remarkable degree. existing both in a variety of different molecular and physical forms. The molecular species. viz.. Sz.S,. S, and S, are in equilibriuni in gaseous sulphur. their proportions varyins with the temperature. The commonest and the most stable allotrope of sulphur at room temperature is known as' . rhombic sulphur or a-sulphur. S,,. In rhomb~csulphur. S, rings are arranged in a way. Fig. 9.2. that gives a rhonibic crystal structure. At 369 K. rhombic sulphur gets convert.ed into monoclinic sulphur or P-sulphur. SF In monoclinic sulphur. Sx rings are arranged in a monoclinic structure. It is .;table between 369 and 392 K. At 392 K it melts to produce a liquid containing Sa molecules. S,. At about 433 K the Sa rings open up and join together Elements of Group 16 into long spiral-chain molecu~ksresulting in a thick viscous liquid, p-sulphur, S,. Liquid sulphur boils at 718 K to give gasews sulphur containing S8 mol~cules,which dissociate to S6, S4, S2 and finally to sulphur atoms at 2273 K. If liquid sulphur at 463 K is poured into cold water, plastic sulphur or y-sulphur is formed. The allotropy of sulphur as a function of temperature is summarised as follows : 1273 K 2273 K -s4-s2-s Selenium, tellurium and polonium also exhjbit allotropy. Amorphous as well as crystalline . .moiecule. forms of selenium and tellurium are known. 9.2.2 Extraction Oxygen is separated from air on a large scale by the fractional distillation of liquid air. It is obtained as a by-prodl~clalong with hydrogen during electrolysis of water for the manufacture heat ............................................ ................................................................. middlepipe. As it comes out from the well, sulphur has a purity of 99.5-99. 9% and contains virtually no As. Se or Te. .................. sulphur from sulphide ores. Selenium and tellurium are obtained in concentrated form from anode mud obtained in the electrdyticrefining of copper. Polonium is made artificially by neutron irradiation of bismuth in a nuclear reactor. 209 1 210 210 Bi, + n -> Bi ------ PO $ e 83 0 83 84 9.2.3 Uses Oxygen is essential for life. Most life processes are based on oxidative metabolism: While the terrestrial beings take up oxygen through respiration, the aquatic plants and animals survive on oxygen dissolved in water. It is an important oxidant used in various energy generation processes through combustion of wood or of fossil fuels like coal, natural gas and petroleum. Rocket fuels have liquid oxygen as the oxidant. Since oxyacetylene and oxy- hydrogen flames have very high temperatures, they are used in cuttillg metals and in welding. Many chemical industries also use oxygen as an oxidant, e.g., in manufacture of,ethylene and propylene oxides, winyl acetate for polymer industry and oxidation of ammonia for manufacture of nitric acid. Ozone which is an allotropic form of oxygen is also a powerful oxidising agent. Ozone undergoes a chatacteristic reaction with unsaturated organic compounds where it attacks a double or triple. bond. It can, therefore, be used for the detection and characterisation of the double or tripre bond. It is also used in the treatment of drinking water. p-Block Elements-11 Sulphur is used in the manufacture of sulphur dioxide, sulphuric acid, gunpowder. n~atche\. fertilisers, drugs, bleaching agents, leather and other products. Large quantities of elemental Detergents are sodium salts of alkyl sulphur are used in the vulcanisation of rubber, in certain pintments and medicines. benzene sulphonic acids where alkyl Sulphuric acid of varying concentrations is used in the manufacture of fertilisers, paints, groups art: linear. Heavy metal salts pigments, dye-stuffs, fibres, plastics, detergents and soaps. It also finds its use in refining of these benzene sulphonic acids are petroleum.