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Group Vi Elements (The Chalcogens)

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Group Vi Elements (The Chalcogens) GROUP VI ELEMENTS (THE CHALCOGENS) Elements are: - Oxygen-O, Sulphur-S, Selenium-Se, Tellurium-Te & Polonium-Po. Valence shell electronic configuration:- ns2np4 Compound formation:- O - S - covalent bonding Se - Te - tend to form ionic compound Po - down the group. Table 1: Some physical properties of Group VI elements. Property O(8) S(16) Se(34) Te(52) Po(84) Electronic [He]2s22p4 [Ne]3s23p4 [Ar]3d104s24p4 [Kr]4d105s25p4 [Xe]4f145d106s26p4 configuration 1st IE (kJmol-1) 1314 1000 941 869 813 Electronegativity 3.5 2.6 2.6 2.0 1.75 Melting pt. (oC) -229 114 221 452 254 Boiling pt (oC) -183 445 685 869 813 Density (gm-3) 1.14 2.07 4.79 6.25 9.4 Electron -141 -200 -195 -190 -183 affinity,E- Ionic radius M2- 1.40 1.85 1.95 2.20 2.30 /Ao Covalent 0.73 1.04 1.17 1.37 1.46 radius/Ao Oxidation states -2,-1,1,2 -2,2,4,6 -2,2,4,6 -2,2,4,6 2,4 Oxygen shows oxidation states of +1 and +2 in oxygen fluorides OF2 and O2F2 Occurrence:- Oxygen is the most abundant of all elements on earth. Dry air contains 20.946% oxygen by volume in the free form. Oxygen forms about 46.6% by weight of the earth’s crust including oceans and the atmosphere. Most of the combined oxygen is in the form of silicate, oxides and water. The abundance of sulphur in the earth’s crust is only 0.03-0.1%. it is often found as free element near volcanic regions. Combined sulphur exists primary in sulphates and sulphides, such as gypsum,CaSO4.2H2O, barite, BaSO4 and Epsom salt, MgSO4.7SO4. The sulphides include galena, PbS, zinc blende, ZnS, chalcopyrite, CuFeS2, iron pyrite, FeS2 and cinnabar, HgS, etc. It also occurs in mineral springs as H2S. Along with C,H,O,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. 1 Oxidation states:- Oxygen exhibits an oxidation state -2 in its compounds. It exhibits positive oxidation state only in a few compounds formed with fluorine, i.e., OF2 and O2F2. The tendency for the formation of divalent anions decreases from sulphur downwards because of the increasing size and decreasing electronegativity of the elements. S, Se, and Te show a tendency for covalence with formal oxidation states +2, +4 and +6 in compounds in which they are combined with more electronegative elements such as oxygen and halogens. In the higher oxidation states of +4 and +6 of these elements; electrons are unpaired and promoted to vacant d orbitals. Note: Group VI elements react with hydrogen to form hydrides and with metals to form binary compounds in which they exhibit -2 oxidation state. Allotropic forms:- All the elements of the group show allotropy. Oxygen: Oxygen exists in two allotropic forms. (i) Dioxygen, O2, is a diatomic gas, paramagnetic in nature. Lewis structure of oxygen molecule with a pair of covalent bonds between two oxygen atoms is inadequate in explaining its paramagnetic nature. x x . x . x xx ... Lewis structure of O2 molecules This structure with all paired electrons is expected to be diamagnetic rather than paramagnetic. Paramagnetism of oxygen can be explained on the basis of MOT.The molecular orbital configuration of O2 molecule can be represented as:- 2 2 2 2 2 2 2 1 1 (1s) ,(1s*) ,(2s) ,(2s*) ,(2px) ,(2py =2pz ),(*2py =*2pz ) The presence of two unpaired electrons in the antibonding orbitals explains the experimentally observed paramagnetic behaviour. (ii) Ozone, O3, the other allotropic form of oxygen is a triatomic, pale blue gas. The earth, in the upper atmosphere is covered by a layer of ozone which protects us from injurious ultraviolet rays coming from the sun. Sulphur: Sulphur displays allotropy to a remarkable degree, existing both in a variety of different molecular and physical forms. However, the structural properties of some of these allotropes are very complex and not well understood. The molecular species, S2, S4, S6 and S8 are in equilibrium in gaseous sulphur, their proportions varying with the temperature. 2 -sulphur:- The commonest and the most stable allotrope of sulphur at room temperature is known as rhombic sulphur or -sulphur or S8 sulphur. It consists of puckered S8 rings shown below. S S S S S o S d(S-S)=2.037A <(SSS)=107.8o S S l bond S ra ed angle ih e d gl S S an .. S dihedral angle=99o The S-S bond in this system contains virtually no character which could theoretically arise from p-d interaction. The extensive allotropy of sulphur arises because there are very small differences in energy between sulphur rings containing six to twelve atoms and “infinite” chain of sulphur atoms. Within the series of six to twelve membered rings, the <SSS bond angle varies from 102o to 108o and the dihedral angle increases from 74o to 100o. Selenium: possesses many of the characteristics of sulphur. E.g. Se atoms can bond to each other to form Se8 molecules. Tellurium: has various allotropic forms. Polonium: No allotropy. Why does the chemistry of oxygen differ from the chemistry of other elements of the group? (i) Oxygen has high electronegativity whereas the other members have lower ENs. (ii) O2 can form hydrogen bond in many compounds while the rest can not. E.g. H2O is liquid due to hydrogen bond while H2S is a gas because it does not have hydrogen bond. (iii) Oxygen can’t expand the octet, sulphur and the other elements can expand their 2- octets using the d orbitals. E.g. SF6, Te(OH)6, TeF8 . 2- 2- 2- They can also form d-p bonds E.g. SO4 , SeO4 , TeO4 , SO2, SO3 etc. 3 Group VI elements can complete octet in different ways: • By gaining two electrons and forming O2-, S2- etc. except polonium which is too metallic for that. Sulphur and the higher elements can only form these ions when reacting with the most electropositive metals. • By forming two covalent bonds as in the hydrides H2O, H2S, H2Po. Here the acidity increases and the stability decreases going down the group. • By loosing four electrons and forming 4-valent cations. However, only Te and Po form these due to the inert pair effect. E.g. in oxides. OXIDES Oxygen reacts practically with all the elements in the periodic table except the lighter noble gases to form binary compounds called oxides. All elements can form more than one oxide of varied compositions, depending on the method of preparation, reaction conditions, etc. The oxides can be classified on the basis of the oxidation state of oxygen in the oxide or on the basis of their chemical nature. Oxides have properties characteristic of ionic (metallic oxides) and covalent (non- metallic oxides) compounds. Categories of oxides:- • Normal oxides contain oxygen in its normal oxidation state of -2.e.g., MgO, O=C=O, etc. 2- • Peroxides contain the peroxide ion, O2 , e.g., Na-O-O-Na, H-O-O-H, etc. - • Superoxides contain O2 ion and are formed by alkali metals, e.g., KO2, CsO2, etc. • Suboxides involve bonds between atoms of the elements in addition to bonds between the element and the oxygen, e.g., O=C=C=C=O. • Mixed oxides are regarded as composed of two simpler oxides, e.g., Pb3O4 (2PbO.PbO2). Normal oxides in which oxygen is in oxidation state 2 (O2-) can be divided into three classes based upon their structure:- • Molecular oxides: E.g., CO2, CO, N2O, NO, N2O3, NO2, N2O5, SO2, SO3, Cl2O7 Form oxides that are volatile and crystallize in structures containing discrete molecules. • Oxides that form giant molecules: E.g¨SiO2, B2O3, BeO. Polymerization of oxides becomes more extensive in the case of the more metallic elements and the more non-metals form oxides which are polymerized to some extent. E.g., M2O3 and M2O5 where M=P, As, Sb. • Ionic oxides: Formed with the most metallic elements. E.g., heavier elements of Group 1and II. 4 A broader classification is based on the reaction of the oxide with water. According to this, oxides are classified as acidic, basic, neutral or amphoteric. • Acidic oxides dissolve in water to give acidic solutions. These react with alkalis forming a salt and water, e.g., CO2, NO2, SO2, etc. • Basic oxides dissolve in water to give alkaline solutions. These react with acids forming salt and water, e.g., Na2O, CaO, MgO, etc. • Neutral oxides have neither acidic nor basic properties and when dissolved in water, they give neutral solutions, e.g., CO, N2O, etc. • Amphoteric oxides show both acidic and basic properties. They react with acids as well as with alkalis to give salt and water, e.g., Al2O3, ZnO, PbO, etc. ZnO + 2HCl ZnCl2 + H2O ZnO + NaOH Na2ZnO2 + H2O Al2O3 + 6HCl 2AlCl3 + 3H2O Al2O3 + 2NaOH 2NaAl(OH)4 Oxygen with other elements of Group VI Oxygen reacts readily with the other elements in this family to form two principal oxides, (which are important stable):- MO2 – dioxides MO3 – trioxides However, mono and other oxides are known. Table2: Oxides of Group VI elements Element Mono-oxide Dioxide Trioxide Others S SO SO2 SO3 S2O, S2O3, S2O7, SO4 Se SeO SeO2 SeO3 Se2O3 Te TeO TeO2 TeO3 - Po PoO PoO2 - - Bond lengths and p-d bonding: The bonds between S and O are much shorter than expected for a single bond in its oxides and therefore, may be considered as double bonds.
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