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THE p-BLOCK ELEMENTS 307 UNIT 11 THE p -BLOCK ELEMENTS...

THE p-BLOCK ELEMENTS 307 UNIT 11 THE p -BLOCK ELEMENTS d he The variation in properties of the p-block elements due to the influence of d and f electrons in the inner core of the heavier is elements makes their chemistry interesting After studying this unit, you will be bl able to appreciate the general trends in the In p-block elements the last electron enters the outermost pu chemistry of p-block elements; p orbital. As we know that the number of p orbitals is three describe the trends in physical and and, therefore, the maximum number of electrons that can chemical properties of group 13 and be accommodated in a set of p orbitals is six. Consequently 14 elements; there are six groups of p–block elements in the periodic be T explain anomalous behaviour of table numbering from 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups. Their valence re boron and carbon; o R 2 1-6 shell electronic configuration is ns np (except for He). describe allotropic forms of carbon; The inner core of the electronic configuration may, know the chemistry of some however, differ. The difference in inner core of elements tt E important compounds of boron, greatly influences their physical properties (such as atomic carbon and silicon; and ionic radii, ionisation enthalpy, etc.) as well as chemical list the important uses of group 13 properties. Consequently, a lot of variation in properties of C and 14 elements and their elements in a group of p-block is observed. The maximum compounds. oxidation state shown by a p-block element is equal to the total number of valence electrons (i.e., the sum of the s- no N and p-electrons). Clearly, the number of possible oxidation states increases towards the right of the periodic table. In addition to this so called group oxidation state, p-block elements may show other oxidation states which normally, © but not necessarily, differ from the total number of valence electrons by unit of two. The important oxidation states exhibited by p-block elements are shown in Table 11.1. In boron, carbon and nitrogen families the group oxidation state is the most stable state for the lighter elements in the group. However, the oxidation state two unit less than the group oxidation state becomes progressively more stable for the heavier elements in each group. The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the ‘inert pair effect’. 307 307 C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd,27.7.6, Reprint16.10.6 27.7.6(reprint)  308 CHEMISTRY Table 11.1 General Electronic Configuration and Oxidation States of p-Block Elements Group 13 14 15 16 17 18 General electronic ns2np1 ns2np2 ns2np3 ns2np4 ns2np5 ns2np6 configuration (1s2 for He) First member of the B C N O F He d group Group he oxidation +3 +4 +5 +6 +7 +8 state Other oxidation +1 +2, – 4 +3, – 3 +4, +2, –2 +5, + 3, +1, –1 +6, +4, +2 is states bl The relative stabilities of these two oxidation The first member of p-block differs from the states – group oxidation state and two unit less remaining members of their corresponding than the group oxidation state – may vary from pu group in two major respects. First is the size group to group and will be discussed at and all other properties which depend on size. appropriate places. Thus, the lightest p-block elements show the It is interesting to note that the non-metals same kind of differences as the lightest s-block be T and metalloids exist only in the p-block of the elements, lithium and beryllium. The second important difference, which applies only to the re periodic table. The non-metallic character of o R elements decreases down the group. In fact the p-block elements, arises from the effect of d- heaviest element in each p-block group is the orbitals in the valence shell of heavier elements most metallic in nature. This change from non- (starting from the third period onwards) and tt E metallic to metallic character brings diversity their lack in second period elements. The in the chemistry of these elements depending second period elements of p-groups starting on the group to which they belong. from boron are restricted to a maximum C covalence of four (using 2s and three 2p In general, non-metals have higher ionisation orbitals). In contrast, the third period elements enthalpies and higher electronegativities than of p-groups with the electronic configuration no N the metals. Hence, in contrast to metals which n 3s23p have the vacant 3d orbitals lying readily form cations, non-metals readily form between the 3p and the 4s levels of energy. anions. The compounds formed by highly Using these d-orbitals the third period reactive non-metals with highly reactive metals elements can expand their covalence above © are generally ionic because of large differences four. For example, while boron forms only in their electronegativities. On the other hand, – 3– [BF 4] , aluminium gives [AlF 6] ion. The compounds formed between non-metals presence of these d-orbitals influences the themselves are largely covalent in character chemistry of the heavier elements in a number because of small differences in their of other ways. The combined effect of size and electronegativities. The change of non-metallic availability of d orbitals considerably to metallic character can be best illustrated by influences the ability of these elements to form the nature of oxides they form. The non-metal π bonds. The first member of a group differs oxides are acidic or neutral whereas metal from the heavier members in its ability to form oxides are basic in nature. pπ - pπ multiple bonds to itself ( e.g., C=C, C≡C, 308 308 C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd,27.7.6, Reprint16.10.6 27.7.6(reprint)  THE p-BLOCK ELEMENTS 309 N≡N) and to other second row elements (e.g., 11.1.1 Electronic Configuration C=O, C=N, C≡N, N=O). This type of π - bonding The outer electronic configuration of these 2 1 is not particularly strong for the heavier elements is ns np. A close look at the p-block elements. The heavier elements do form electronic configuration suggests that while π bonds but this involves d orbitals (dπ – pπ boron and aluminium have noble gas or dπ –dπ ). As the d orbitals are of higher core, gallium and indium have noble gas plus energy than the p orbitals, they contribute less 10 d-electrons, and thallium has noble gas to the overall stability of molecules than does plus 14 f- electrons plus 10 d-electron cores. pπ - pπ bonding of the second row elements. Thus, the electronic structures of these d However, the coordination number in species elements are more complex than for the first of heavier elements may be higher than for two groups of elements discussed in unit 10. he the first element in the same oxidation state. This difference in electronic structures affects For example, in +5 oxidation state both N and the other properties and consequently the – P form oxoanions : NO3 (three-coordination chemistry of all the elements of this group. with π – bond involving one nitrogen p-orbital) 11.1.2 Atomic Radii is and PO34− (four-coordination involving s, p and On moving down the group, for each successive d orbitals contributing to the π – bond). In member one extra shell of electrons is added this unit we will study the chemistry of group and, therefore, atomic radius is expected to bl 13 and 14 elements of the periodic table. increase. However, a deviation can be seen. Atomic radius of Ga is less than that of Al. This 11.1 GROUP 13 ELEMENTS: THE BORON can be understood from the variation in the pu FAMILY This group elements show a wide variation in inner core of the electronic configuration. The presence of additional 10 d-electrons offer properties. Boron is a typical non-metal, only poor screening effect (Unit 2) for the outer be T aluminium is a metal but shows many electrons from the increased nuclear charge in re chemical similarities to boron, and gallium, gallium. Consequently, the atomic radius of o R indium and thallium are almost exclusively gallium (135 pm) is less than that of metallic in character. aluminium (143 pm). Boron is a fairly rare element, mainly tt E 11.1.3 Ionization Enthalpy occurs as orthoboric acid, (H3BO3), borax, The ionisation enthalpy values as expected Na2B4O7·10H2O, and kernite, Na2B4O7·4H2O. from the general trends do not decrease C In India borax occurs in Puga Valley (Ladakh) smoothly down the group. The decrease from and Sambhar Lake (Rajasthan). The B to Al is associated with increase in size. The abundance of boron in earth crust is less than observed discontinuity in the ionisation no N 0.0001% by mass. There are two isotopic 10 11 enthalpy values between Al and Ga, and forms of boron B (19%) and B (81%). between In and Tl are due to inability of d- and Aluminium is the most abundant metal and f-electrons ,which have low screening effect, to the third most abundant element in the earth’s compensate the increase in nuclear charge. © crust (8.3% by mass) after oxygen (45.5%) and Si (27.7%). Bauxite, Al2O3. 2H2O and cryolite, The order of ionisation enthalpies, as Na 3 AlF 6 are the important minerals of expected, is Δi H1 Pb. Carbon provides one of the best examples of allotropy. Three important allotropes of carbon are diamond, graphite pu and fullerenes. The members of the carbon family mainly exhibit +4 and +2 oxidation states; compouds in +4 oxidation states are generally covalent in nature. The tendency to show +2 oxidation state increases among heavier elements. Lead in +2 state is stable be T whereas in +4 oxidation state it is a strong oxidising agent. Carbon also exhibits negative oxidation states. It forms two important oxides: CO and CO2. Carbon monoxide is neutral re whereas CO2 is acidic in nature. Carbon monoxide having lone pair of electrons on C o R forms metal carbonyls. It is deadly poisonous due to higher stability of its haemoglobin complex as compared to that of oxyhaemoglobin complex. Carbon dioxide as such is not toxic. However, increased content of CO2 in atmosphere due to combustion of fossil fuels tt E and decomposition of limestone is feared to cause increase in ‘green house effect’. This, in turn, raises the temperature of the atmosphere and causes serious complications. Silica, silicates and silicones are important class of compounds and find applications C in industry and technology. no N EXERCISES 11.1 Discuss the pattern of variation in the oxidation states of (i) B to Tl and (ii) C to Pb. © 11.2 How can you explain higher stability of BCl3 as compared to TlCl3 ? 11.3 Why does boron triflouride behave as a Lewis acid ? 11.4 Consider the compounds, BCl 3 and CCl 4. How will they behave with water ? Justify. 11.5 Is boric acid a protic acid ? Explain. 11.6 Explain what happens when boric acid is heated. 11.7 Describe the shapes of BF3 and BH4–. Assign the hybridisation of boron in these species. 11.8 Write reactions to justify amphoteric nature of aluminium. 323 C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, Reprint 27.7.6  324 CHEMISTRY 11.9 What are electron deficient compounds ? Are BCl 3 and SiCl 4 electron deficient species ? Explain. 2– – 11.10 Write the resonance structures of CO3 and HCO3. 2– 11.11 What is the state of hybridisation of carbon in (a) CO 3 (b) diamond (c) graphite? 11.12 Explain the difference in properties of diamond and graphite on the basis of their structures. 11.13 Rationalise the given statements and give chemical reactions : d Lead(II) chloride reacts with Cl2 to give PbCl4. Lead(IV) chloride is highly unstable towards heat. he Lead is known not to form an iodide, PbI4. – 11.14 Suggest reasons why the B–F bond lengths in BF 3 (130 pm) and BF 4 (143 pm) differ. 11.15 If B–Cl bond has a dipole moment, explain why BCl3 molecule has zero dipole moment. is 11.16 Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous BF3 is bubbled through. Give reasons. bl 11.17 Suggest a reason as to why CO is poisonous. pu 11.18 How is excessive content of CO2 responsible for global warming ? 11.19 Explain structures of diborane and boric acid. 11.20 What happens when (a) Borax is heated strongly, be T (b) Boric acid is added to water, re (c) Aluminium is treated with dilute NaOH, o R (d) BF3 is reacted with ammonia ? 11.21 Explain the following reactions tt E (a) Silicon is heated with methyl chloride at high temperature in the presence of copper; (b) Silicon dioxide is treated with hydrogen fluoride; C (c) CO is heated with ZnO; (d) Hydrated alumina is treated with aqueous NaOH solution. 11.22 Give reasons : no N (i) Conc. HNO3 can be transported in aluminium container. (ii) A mixture of dilute NaOH and aluminium pieces is used to open drain. (iii) Graphite is used as lubricant. © (iv) Diamond is used as an abrasive. (v) Aluminium alloys are used to make aircraft body. (vi) Aluminium utensils should not be kept in water overnight. (vii) Aluminium wire is used to make transmission cables. 11.23 Explain why is there a phenomenal decrease in ionization enthalpy from carbon to silicon ? 11.24 How would you explain the lower atomic radius of Ga as compared to Al ? 11.25 What are allotropes? Sketch the structure of two allotropes of carbon namely diamond and graphite. What is the impact of structure on physical properties of two allotropes? 324 C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, Reprint 27.7.6  THE p-BLOCK ELEMENTS 325 11.26 (a) Classify following oxides as neutral, acidic, basic or amphoteric: CO, B2O3, SiO2, CO2, Al2O3, PbO2, Tl2O3 (b) Write suitable chemical equations to show their nature. 11.27 In some of the reactions thallium resembles aluminium, whereas in others it resembles with group I metals. Support this statement by giving some evidences. 11.28 When metal X is treated with sodium hydroxide, a white precipitate (A) is obtained, which is soluble in excess of NaOH to give soluble complex (B). Compound (A) is soluble in dilute HCl to form compound (C). The compound d (A) when heated strongly gives (D), which is used to extract metal. Identify (X), (A), (B), (C) and (D). Write suitable equations to support their identities. he 11.29 What do you understand by (a) inert pair effect (b) allotropy and (c) catenation? 11.30 A certain salt X, gives the following results. (i) Its aqueous solution is alkaline to litmus. is (ii) It swells up to a glassy material Y on strong heating. (iii) When conc. H2SO4 is added to a hot solution of X,white crystal of an acid Z separates out. bl Write equations for all the above reactions and identify X, Y and Z. 11.31 Write balanced equations for: (i) BF3 + LiH → pu (ii) B2H6 + H2O → (iii) NaH + B2H6 → be T Δ (iv) H3BO3 ⎯ → (v) Al + NaOH → re o R (vi) B2H6 + NH3 → 11.32. Give one method for industrial preparation and one for laboratory preparation of CO and CO2 each. tt E 11.33 An aqueous solution of borax is (a) neutral (b) amphoteric C (c) basic (d) acidic 11.34 Boric acid is polymeric due to (a) its acidic nature (b) the presence of hydrogen bonds no N (c) its monobasic nature (d) its geometry 11.35 The type of hybridisation of boron in diborane is (a) sp (b) sp2 (c) sp3 (d) dsp2 11.36 Thermodynamically the most stable form of carbon is © (a) diamond (b) graphite (c) fullerenes (d) coal 11.37 Elements of group 14 (a) exhibit oxidation state of +4 only (b) exhibit oxidation state of +2 and +4 (c) form M2– and M4+ ions (d) form M2+ and M4+ ions 11.38 If the starting material for the manufacture of silicones is RSiCl3, write the structure of the product formed. 325 325 C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, 27.7.6, C:\ChemistryXI\Unit-11\Unit-11-F\Unit-11(reprint).pmd, Reprint16.10.6 27.7.6 (reprint)