1. THE GROUP 13 ELEMENTS Include boron, aluminum, gallium, indium and thallium. Boron is the only nonmetal in the group many of their compounds of the elements are electron deficient and act as Lewis acids. Aluminum is a metalloid And gallium, indium and thallium are metals. He LiBeBCNOFNe MgAlSi CaGaGe SrInSn BaTlPb Ra 1213 1415 16 170

2. Properties of Elements BAlGaInTl Covalent radius/pm80125 150155 Metallic radius/pm143141166171 Ionic radius(M +3 )/pm2753629498 Melting point/°C230066030157304 Boiling point/°C39302470240020001460 1 st I.E.I 1 /kj.mol -1 799577579558 590 2 nd I.E.I 2 /kj.mol -1 2427 1817 1979 1821 1971 3 rd I.E.I 3 /kj.mol -1 3660 2745 2963 2704 2878 Electron affinity, E a /kj.mol -1 26.7 42.5 28.9 Pauling electronegativity2.01.51.61.71.8 E ө (M 3+ ;M)/V-0.89-1.68-0.53-0.34+1.26 Page 288 Sh&At

3. Trends from the table Covalent, metallic and ionic radii increases from B to Tl. Electronegativity: Ga is more electronegative than Al due to the alternation effect. (consequence of increased nuclear charges of the 4p elements due to the presence of their poorly shielding 3d electrons. Distinct chemical properties from those of other elements in a group: - Boron forms acidic oxides, B 2 O 3, and aluminium forms amphoteric oxide, Al 2 O 3 - Boron forms polymeric oxide structure. - Boron form flammable, gaseous hydrides, aluminium hydride is a solid - Boron is electron deficient and making all its neutral compounds Lewis acids. Electronic configuration – ns 2 np 1 Generally exhibit +3 oxidation state and changes to +1 as the group is descended.

4. ElementSymbol I E (kJ.mol -1 )  Electronic Configuration Oxidation state BoronB8012.0[He]2s 2 2p 1 III* AluminiumAl5771.5[Ne]3s 2 3p 1 (1) III* GalliumGa5791.6[Ar]3d 10 4s 2 4p 1 I III* IndiumIn5581.7[Kr]4d 10 5s 2 5p 1 I III* ThalliumTl5901.8[Xe]4f 14 5d 10 6s 2 6p 1 I*III

5. MEAN BOND ENTHALPIES (kJ/mol -1 ) HFClBrI Boron334757536423220 Aluminium284664511444370 Gallium274577481444339 Indium243506439414331 Thallium188445372334272 Occurrence and recovery Al – most abundant and Tl and In are least abundant. B:- Borax; Na 2 B 4 O 5 (OH) 4.8H 2 O and kernite; Na 2 B 4 O 5 (OH) 4.2H 2 O; Borax - Na 2 B 4 O 5 (OH) 4.8H 2 O → boric acid, B(OH) 3 → boron oxide, B 2 O 3 →reduced with Mg, hydrofluoric acid, HF Pure boron is produced by reduction of BBr 3 vapour with H 2 : 2 BBr 3 (g) + 3 H 2 (g)→ 2 B(g) + 6 HBr(g) Al:- clays and aluminosilicate minerals and commercially as bauxite. Gallium oxide occurs naturally as an impurity in bauxite and indium is obtained in the Pb and Zn ores. Thallium compounds are found in the flue dust.

6. Aluminium alloys Al is the widely used nonferrous metal, it’s light, has high electrical and thermal Conductivity and high reflectance. Small percentages (less than 2 %) of other metal gives desirable weight to strength ratio. Al/Mn – most widely used and contains Fe and Si in traces to give added strength and hardness Al/Mg – have good ductility and corrosion resistant but less strong. Al/Si – have good fluidity and used in welding wires. Al/Li – have very low density and high elasticity. Al/Cu/Mg – used in making aircrafts. Al/Cu – used in satellites and space vehicles. Other uses of Al alloys include building and construction – walls, gutters, window frames, roofs and doors, aerosol cans, drink cans, sheets and foils and cooking utensils – toys, tools, refrigerators, cosmetic tubes and airconditioning units and in cars – body panels, engine blocks, wheels, bumpers and radiators.

7. Production of Aluminium (as in Alusaf) First step: Purification of the ore – Bayer’s process. Bauxite is crushed and digested with caustic soda under pressure. Al 2 O 3 dissolves to form sodium aluminate.  The solution is diluted thereby impurities like TiO 2, sodium aluminium silicate and iron(III) oxide precipitate out. They are filtered out.  CO 2 is bubbled through the liquor to reduce the pH to enable Al(OH) 3 precipitate and then filtered and washed.  Al(OH) 3 is calcined at ~1300°C to give pure Al 2 O 3 + 3H 2 O.

8. Second step: Smelting process This is done by electrolysis using the Hall-Héroult process. Al 2 O 3 is dissolved in molten cryolite, Na 3 [AlF 6 ] and electrolysed in a graphite- lined steel tank which serves as the cathode and carbon as anode. Calcium fluoride (fluospar) is added to lower the m.p. Molten aluminium (m.p. 600) is drained from the bottom of the cell at intervals. Cryolite is made synthetically to augment the mined cryolite. 2Al(OH) 3 + 3NaOH + 6HF  Na 3 [AlF 6 ] + 6H 2 O (Cryolite improves the electrical conductivity of the cell as Al 2 O 3 is a poor conductor. It also serves as an added impurity and lowers the m.p. of the mixture to about 900°C).  Typical electrolyte composition ranges are Na 3 [AlF 6 ] (80-85%), CaF 2 (5-7%), AlF 3 (5-7%), Al 2 O 3 (2-8% - intermittently recharged).  The following dissociations occur;

9. Carbon anode burns to CO 2, so the anode is replaced periodically. Some aluminium fog dispersed in the electrolyte, reduces CO2 to CO. Exit gases contain about 30% CO. The overall anode reaction seems to be Al 2 O 2 F 4 2- + 2AlF 6 3- + C  4AlF 4 - + CO 2 + 4e and at the cathode AlF 6 3- + 3e  Al + 6F - Overall cell reaction: 3Al 2 O 2 F 4 2- + 10AlF 6 3- + C  12AlF 4 - + 3CO 2 + 4Al + 24F -  Al is preferentially discharged at the cathode and oxygen at the anode.

11. BORON COMPOUNDS HYDRIDES Boron hydride compounds – Borane, the simplest form is diborane – B 2 H 6 its structure is described as 2c,2e and 3c,2e bonds; can be prepared by metathesis of boron halides with either LiAlH 4 or LiBH 4 in ether. 3 LiBH 4 (et) + 4 BF 3 (et) = 2 B 2 H 6 (g) + 3 LiBF 4 (et) All the boranes are electron deficient, colourless and diamagnetic. Higher boranes are classified according to their electron count: TypeFormulaskeletal electron pairs Examples ClosoB n H n 2- n + 1B 5 H 5 2- to B 12 H 12 2- Nido B n H n+4 n + 2B 2 H 6, B 5 H 9, B 6 H 10 ArachnoB n H n + 6 n + 3B 4 H 10, B 5 H 11 HyphoB n H n + 8 n + 4none Figure 12.12 page 304

12. Structure of diborane Wade’s Rules: established by Kenneth Wade in the 1970s based on correlation between the number of electrons, the formula and the shape of the molecules. This apply to a class of polyhedra called deltahedra because they are made up of triangular faces resembling Δ. For molecular and anionic boranes - predict shapes of molecule or anion from its formula. B-H bonds – 2c-2e B-H-B bonds – 3c-2e Diborane, like all boranes, is electron-deficient. There are 12 electrons (6 from H and 3 each from B). The four B-H bonds use 8 electrons, leaving 2 electrons each for the B-H-B bonds. The B-H-B bonds are therefore electron –deficient (short of 4 electrons)

13. Characteristics reactions of boranes and borohydrides Cleavage of BH 2 unit from diborane or tetraborane by NH 3. Deprotonation of large boron hydrides by bases. Reaction of boron hydrides with borohydride ions to produce larger borohydride anions. Fridel-Crafts type substitution for hydrogen in pentaborane and some larger boron hydrides

14. BORON TRIHALIDES Are useful Lewis acids as a result of incomplete octet, with BCl 3 stronger than BF 3. They consist of trigonal planar BX 3 molecules. BCl 3, BF 3 are gases; BBr 3 is a volatile liquid and BI 3 is a solid. BX 3 B(NH 2 ) 3 RNH 2 B(OH) 3 H2OH2O B(OR) 3 ROH X 3 BPR 3 PR 3 X 3 BSR 2 SR 2 X 3 BNR 3 NR 3 Complex formation Protolysis REACTIONS OF BX 3 COMPOUNDS

15. BORON OXYGEN COMPOUNDS Important oxides – B 2 O 3, polyborates and borosilicate glasses. 2 B(OH) 3 (s) Δ→ B 2 O 3 (s) + 3 H 2 O(l) An example of cyclic polyborate anion, [B 3 O 6 ] 3- - three coordinate B atom and [B(OH) 4 ] - four coordinate B atom. Polyborates form by sharing O atom with the neighboring B atom. The rapid cooling of molten B 2 O 3 lead to the formation of borate glasses. Sodium perborate, commonly written as NaBO 3.4H 2 O contains peroxide ion – O 2 2-, and hence the accurate formula is Na 2 [B 2 (O 2 ) 2 (OH) 4 ].6H 2 O. It is used as a bleach in laundry powders, automatic dishwashing powders and whitening toothpaste.

16. LEWIS ACIDITY Diborane and other hydrides act as Lewis acids and cleaved by reaction with Lewis bases; B 2 H 6 + 2 N(CH 3 ) 3 → 2 H 3 B-N(CH 3 ) 3 Symmetric cleavage B 2 H 6 + 2NH 3 → [BH 4 ] - + [BH 2 (NH 3 ) 2 ] + Unsymmetric cleavage HYDROBORATION, METAL BORIDES – SUPERCONDUCTING ABILITY OF MgB 2 METALLABORANES, CARBORANES SELF STUDY

17. COMPOUNDS WITH OTHER ELEMENTS – Al, Ga, In AND Tl HYDRIDES LiAlH 4 and LiGaH 4 are good precursors for metal hydride complexes, MH 3 L 2 and sources for H - ion for the metalloid hydrides – SiH 4. Aluminium hydride, AlH 3 is a solid similar to the s-block metal hydrides although not readily available. Pure Ga 2 H 6 has been prepared recently and the hydrides of Tl and In are very unstable. Some of the reactions involving the hydrides: - 4 LiH + ECl 3 → (Δ, ether) LiEH 4 + LiCl (E = Al, Ga) - LiAlH 4 + SiCl 4 → (THF) LiAlCl 4 + SiH 4 - LiEH 4 + [(CH 3 ) 3 NH]Cl → (CH 3 ) 3 N-EH 3 + LiCl + H 2 (E = Al, Ga) - (CH 3 ) 3 N-EH 3 + N(CH 3 ) 3 → ((CH 3 ) 3 N) 2 EH 3 + LiCl + H 2 (E = Al, Ga)

18. HALIDES AND TRIHALIDES All elements form trihalides in their +3 oxidation state, however +1 becomes Common with Tl forming a stable monohalide. Trihalides of Al, Ga and In are Lewis acids. Generally prepared by reaction of the electropositive metal with hydrogen halides such as HCl, HBr gases. 2 Al (s) + 6 HCl (g) → (100 °C) AlCl 3 (s) + 3 H 2 (g) Trifluorides are mechanically harder than others and they have high melting points and sublimation enthalpies and have low solubility. Their reactivity towards most donors is low (not Lewis acids) and despite that AlF 3 and GaF 3 form slats such as Na 3 AlF 6 (Cryolite) – used as a solvent for bauxite in the production of aluminium) and Na 3 GaF 6.

19. Thallium trihalides are less stable than those of light congeners. The triiodide, TlI3 is a compound of +1 and +3 as it contains the I3- not the I- ion. Confirmed by the standard electrode potentials which indicates that Tl(III) is rapidly reduced to Tl(I) by iodide: Tl 3+ (aq) + 2 e - → Tl + (aq) E Ө = +1.26 V I 3- (aq) + 2 e - → 3 I - (aq) E Ө = +0.55 V However, in excess iodide Tl(III) is stabilized by the formation of the complex; TlI 3 (s) + I - → TlI 4 - (aq) Generally the +1 oxidation state becomes more stable progressively from Al to Tl. (Read more in page 310)

20. OXYGEN COMPOUNDS Al and Ga form α and β forms of oxide in which the elements are in their +3 oxidation state; Tl forms an oxide in which it’s in +1 oxidation state and a peroxide. α-alumina, Al 2 O 3 is the most stable form and is very hard and a refractory material. In mineral form called corundum and as a gemstone is called sapphire, ruby, emerald or amethyst depending o the amount of metal ion impurities. Dehydration of Al(OH) 3 at temperatures below 900 °C result in the formation of γ-alumina, which is metastable polycrystalline form with a defect spinel structure. The α and γ forms of Ga 2 O 3 have the same structureas their Al analoques. The metastable form is β-Ga 2 O 3, which has a ccp structure with Ga(III) in distorted octahedral and tetrahedral sites. In forms In 2 O 3 and Tl forms Tl(I) oxide and peroxide, Ti 2 O and Tl 2 O 2. The elements all form a series of salts called alums – MAl(SO 4 ) 2.12H 2 O where M is univalent cation such as Na + ; K + ; Rb + ; Cs + ; Tl + or NH 4 +. Alums are thought of double salts containing the hydrated trivalent cation [Al(H 2 O) 6 ] 3+. The remaining water molecules form hydrogen bonds with the sulfate and the cations.

21. +3 Ox. StateB2O3B2O3 Al 2 O 3 Ga 2 O 3 In 2 O 3 Tl 2 O 3 acidicamphoteric basic +1 Ox. StateTl 2 O basic Al 2 O 3 and Al(OH) 3 are amphoteric  Ga 2 O 3 and Ga(OH) 3 are amphoteric and dissolve in alkali to form gallates.  In 2 O 3 and Tl 2 O 3 are completely basic and the metals form neither hydrates nor hydroxides.  TlOH is a strong base and is soluble in water, like Group 1A hydroxide.  Tl + compounds are extremely poisonous and in large doses, can cause death.

22. SULFIDE COMPOUNDS The only sulfide of Al, Al 2 S 3, which is prepared by direct reaction of the elements at elevated temperatures: 2 Al(s) + 3 S(s) → (Δ) Al 2 S 3 (s). It is rapidly hydrolyzed in aqueous solution; Al 2 S 3 (s) + 6H 2 O(l) → 2Al(OH) 3 (s) + 3H 2 S(g) Al 2 S 3, exists in three different forms – α, β and γ forms. The structure of α and β forms are based on wurtzite structure in α–Al 2 S 3 the S 2- ions are hexagonal close packed and the Al 3+ ions occupy two-thirds of the tetrahedral sites randomly. The γ-form adopts the same structure as γ-Al 2 O 3. The sulfides of Ga, Tl and In are numerous and varied than those of Al and adopt many different structural types. Al, In and Ga react with P, As and Sb to form materials that are semiconductors

23. SulfideStructureMaterialBand gap (E g ; eV) GaSLayer structure with Ga-Ga bonds GaAs1.35 α-Ga 2 S 3 Defect wurtzite structure (hexagonal) GaSb0.67 γ-Ga 2 S 3 Defect sphalerite structure (cubic) InAs0.36 InSLayer structure In-In bondsInSb0.163 β-In 2 S 3 Defect spinel (γ-Al 2 O 3 )Si1.107 TlSChains of edge-shared Tl III S 4 tetrahedra Tl 4 S 3 Chains of Tl III S 4 and Tl I Tl III S 3 tetrahedra