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Home , Allotropy

GROUP VI ELEMENTS (THE )

Elements are: - -O, Sulphur-S, -Se, -Te & -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, 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 minerals.

1 Oxidation states:- Oxygen exhibits an -2 in its compounds. It exhibits positive oxidation state only in a few compounds formed with , 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 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 , paramagnetic in nature. Lewis structure of oxygen molecule with a pair of covalent bonds between two oxygen 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. 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) , 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 .

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

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 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 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 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. Along with a sigma bond between S and O, a - bond is also formed by overlap of an oxygen p-orbital and sulphur d-orbital forming a p-d bond. Oxygen and sulphur have comparable energy and the size of p and d orbitals and, therefore, have effective p-d overlap. But going from Se to Te, weaker bonds are formed because of difference in size and energy of the p and d orbitals.

5 Structures:

Dioxides: SO2 is a gas; SeO2 is a white volatile while TeO2 is a non-volatile white solid. Gaseous SO2 and SeO2 have discrete symmetrical molecules which are bent or angular. On solidification SeO2 forms long polymeric chains, in which the SeO2 units consist of bridging oxygen atoms.

O O O O .. S Se Se Se .. .. .O. .O. O O O

SO2 molecule SeO2

TeO2 crystallizes in rutile structure.

Trioxides: SO3 in gaseous form is monomeric planar molecule.

O

S

O O

SO3 molecule Solid SO3 is polymorphic which exists in several forms, amongst which three are the stable ones. • When the vapour is condensed at very low , a trimeric cyclic- or ice-like is obtained (mpt. 17oC). Each S is almost tetrahedrally surrounded by four oxygen atoms. It consists of rings of S3O9 molecules made up of SO4 tetrahedral, each sharing an oxygen atom. • In the presence of moisture the trimer polymerizes further to long spiral chain molecules of a fibrous- or asbestos-like form (mpt. 32.5oC). It consists of infinite chains of SO4 tetrahedral also sharing oxygen atoms. • -form is quite similar to the latter form but the chains are cross-linked by some -O-O- bridges (mpt. 62.6oC). It has a colloidal appearance, but no structural information is available.

SeO3 appears to exist in two crystalline forms. Little is known of the detailed structure of any of the forms of Se or Te trioxides.

6 HALIDES

Group VI elements form a number of compounds with halogens.

Table 3: Binary halides of Group VI elements. Elements Fluorides Chlorides* Bromides* Iodides* O OF2, O2F2 Cl2O, ClO2, Br2O, BrO2, I2O4, I2O5, I2O9 Cl2O6, Cl2O7 BrO3 S S2F2, SF2, SF4, S2Cl2, SCl2, S2Br2 - SF6, S2F10 SCl4 Se Se2F2, SeF4, Se2Cl2, SeCl2, Se2Br2, SeBr2, - SeF6 SeCl4 SeBr4 Te TeF4, TeF6 TeCl2, TeCl4 TeBr2, TeBr4 TeI4 Po - PoCl2, PoCl4 PoBr2, PoBr4 PoI4 (*) In case of oxygen, oxides.

S, Se and Te form M2X2 (Monohalides), MX2 (Dihalides), MX4 (Tetrahalides) and MX6 (Hexahalides). Also M2X10 (where M = S, Te) are known.

The halogen compounds of S, Se, Te and Po are called halides. For oxygen, only the fluoro compounds are called as fluorides while the chloro, bromo and iodo compounds are designated as oxides. This is because of the high electronegativity of oxygen which is exceeded only by fluorine.

Fluorine forms compounds in which the Group VI element occurs in the highest oxidation state. And all these elements form hexafluorides, no other hexahalogen derivatives are known.

HYDRIDES All the elements of this group form simple volatile binary hydrides of the type H2M (where M = O, S, Se, Te & Po). Oxygen: forms H2O (water) and H2O2, (hydrogen peroxide) Sulphur: forms the most extensive series of catenated hydrides such as H2S, H2S2, H2S3, H2S4, etc. Table 4: Some physical properties of binary hydrides Property H2O (liquid) H2S (gas) H2Se (gas) H2Te (gas) Bond energy M-H 467 347 276 - (kJmol-1) Melting pt. (oC) 0 -85.6 -65.5 -51.0 Boiling Pt. (oC) 100 -60.7 -41.5 -1.8 Bond angle 104.5o 92.2o 91.0o 89.5o

• Acidity in water increases. H2O is neutral. • Reducing power increases

7 Structure of H2M:

. .. . M H H 3 The hydride molecules are angular in shape. In case of H2O, oxygen is supposed to be sp hybridized with two lone pairs occupying two positions on the tetrahedron. The distortion from the tetrahedral angle (109o28) is supposed to be due to stronger repulsion between lone pairs of electrons compressing the bond angle. In the case of other hydrides, bond angles are close to 90o suggesting that almost pure p-orbitals are involved in bonding to hydrogen.

USES

Oxygen: • Oxygen is essential for life. Most life 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 oxidant. Many chemical industries also use oxygen as an oxidant, e.g., in manufacture of ethylene and propylene oxides, vinyl acetate for polymer industry and oxidation of ammonia for manufacture of nitric acid. • Since oxyacetylene and oxyhydrogen flames have very high temperatures, they are used in cutting metals and in welding. • Ozone is also a powerful oxidizing agent. It undergoes a characteristic reaction with unsaturated organic compounds where it attacks a double or triple bond.(It can therefore, be used for the detection and characterization of the double or triple bond). It is also used in the treatment of drinking water.

Sulphur: • Is used in the manufacture of sulphur dioxide, sulphuric acid, gunpowder, matches, fertilizers, drugs, bleaching agents, leather and other products. • Large quantities of elemental sulphur are used in the vulcanization of rubber, in certain ointments and medicines. • Sulphuric acid, H2SO4, of varying concentrations is used in the manufacture of fertilizers, paints, pigments, dye-stuffs, fibres, plastics, detergents and soaps. It also finds its use in refining petroleum. • -sulphur compounds have superconducting properties. A binary polymer, (SN)x, exhibits metallic characteristics and becomes superconducting below 0.33K.

Selenium: • Uses include photocopying process of xerography, decolorisation of glasses and as a catalytic agent particularly in the isomerisation of certain petroleum products.

8 • Selenium dioxide is used as an oxidizing agent in organic reactions.

Tellurium: • Is used primarily as an additive to steel to increase its ductility. • It is also used as an additive to some catalysts, used in cracking of petroleum, as a colouring material for glasses and as an additive to to increase its strength and make it corrosion resistant.

Polonium: • Polonium-210 is used mainly for the production of neutron sources; for these, polonium is alloyed with elements such as .

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