P-block Elements General Points: • • • ns2 np1-6 Sometimes show OS 2 units less than group OS due to ‘inert pair effect’ Elements from period 3 onwards can use empty d-orbitals to form more π bonds. Boron Family: • • • • • • • • • • • • • • • • • ns2 np1 Atomic Radius increases down group. Exception: Ga(135pm)<Al(143pm) ∆H1: B > Tl > In > Ga > Al ∆H2: B > Ga > Tl > In > Al ∆H3: B > Ga > Tl > Al > In Electronegativity: B > Tl > In > Ga > Al Boron is hard, black, non-metallic; has high melting point Boron has many allotropic forms Others are soft metals with low mp and high conductivity. Density increases down group Stability of +1 OS to +3 OS: Al < Ga < In < Tl Tl+1 is predominant over Tl+3; +3 is highly oxidising in character Compounds formed in +3 state are electron deficient (6valence e-) – tend to be Lewis acids +3 state compounds are hydrolysed into [M(OH)4]- . Hybridisation: sp3 AlCl3 forms octahedral [Al(H2O)6]+3, hybridisation sp3d2 Al forms protective oxide layer on reaction with air B oxides are acidic; Al and Ga oxides are amphoteric; In and Tl oxides are basic Boron Compounds: 1) Borax: • • • White crystalline solid Na2B4O7.10H2O or Na2[B4O58(OH)4].8H2O Metaborates of transition elements have characteristic colours – used to identify the metal 2) Orthoboric acid: • • • • • White crystalline solid H3BO3 or B(OH)3 Formed by hydrolysis of most boron compounds Boric acid is a weak monobasic acid that accepts e- from hydroxyl ion On heating forms Metaboric acid; on further heating forms Boric oxide 3) Diborane: • • • • • • • B2H6 Colourless, highly toxic with bp=180K Catches fire on exposure to air, gives large amt of energy Four terminal H-atoms and two B-atoms lie in one plane. Intermediate H-atoms lie above and below this plane Terminal bonds are 2c-2e- and bridge bonds are 3c-2e- bonds Boron also forms borohydrides like LiBH4 and NaBH4, which are used as reducing agents in organic synthesis. They are also useful starting materials for preparing other metal borohydrides. Uses of Boron, Aluminium, and their compounds: • • • • • • • Boron fibres are used in bullet-proof vests and light composite materials for aircraft B-10 isotope is used in nuclear industry as protective shields and control rods due to its ability to absorb neutrons Borax and Boric acid are used to manufacture heat-resistant glass, glass-wool and fibreglass Borax in used as flux for soldering metals, for heat-, scratch- and stain-resistant glazed coatings, and in medicinal soaps Aq Orthoboric acid is used as mild antiseptic Aluminium and its alloys are used to make pipes, tubes, rods, wires, plates, foils etc They are used in packing, aeroplanes, utensils, construction, and transportation. But now it is reduced due to its toxic nature Group 14: • • • • • • • • • • • • • • • • • • • • • • ns2 np2 Atomic radius increases down the group ∆H1: C > Si > Ge > Pb > Sn ∆H2: C > Si > Ge > Pb > Sn ∆H3: C > Si > Ge > Pb > Sn ∆H4: C > Si > Ge > Pb > Sn Electronegativity: C > Pb > Sn ≈ Ge ≈ Si C, Si are non-metals; Ge is metalloid; Sn, Pb are metals mp and bp of group-14 are higher than corresponding group-13 elements Exhibit +4 and +2 OS. Ge < Sn < Pb tendency to exhibit +2 due to inability of ns2 participating Stability of +2: Ge < Sn < Pb Sn is reducing agent in +2 and Pb is oxidising agent in +4 Electron precise molecules – do not accept/donate electrons Period 3 and above form SiF62-, [GeCl6]2-, etc with sp3d2 hybridisation React with oxygen forming MO and MO2. Acidic oxides – CO2, SiO2, GeO, GeO2 Amphoteric – SnO, SnO2, PbO, PbO2 Sn decomposes steam to form H2 gas and SnO2, others don’t react with water Pb forms protective layer with water so no reaction React with halogens forming MX2 and MX4. Except C all react directly with halogens SnF4 and PbF4 are ionic, rest are covalent with sp3 hybridisation Stability: GeX4>GeX2, PbX2>PbX4 (stability of dihalides increases down group) Anomalous Properties: • • • • C has high electronegativity and high ∆Hi due to small size Heavier elements due not form pπ-pπ bonds because orbitals are too large C-C bonds are very strong, and C tends to forms long chains –> catenation. Order of catenation: C >> Si > Ge ≈ Sn. Pb does not show catenation. Allotropic forms of Carbon: 1) Diamond • • • • • Has a crystalline lattice All C-atoms are sp3 C-C bond length = 154pm Hardest substance as it is difficult to break extended covalent bonding Used in making dyes and tungsten filaments for bulbs 2) Graphite • • • • • Layered structure held by van der Waals’ forces Distance between layers=340pm, C-C bond length in layer=141.5pm All C-atoms are sp2 hybridised (3σ, 1π) Electrons are delocalised throughout, so graphite conducts electricity well Graphite cleaves easily between the layers, thus is slippery and soft. Hence it is used as lubricant for high temperatures where oil cannot be used. 3) Fullerenes • • • • • • • Fullerenes are the only pure form of carbon because of smooth structure without dangling bonds. Buckminsterfullerene – C60 – has 12 pentagons and 20 hexagons All C-atoms sp2 hybridised 3π bonds, 1 delocalised e- which gives aromatic character C-C length=143.5pm, C=C length=138.3pm Spherical fullerenes are also called Bucky Balls Carbon black, coke, charcoal are impure forms of graphite or fullerenes. Formed by heating wood or coal at high temperature in the absence of air Uses of Carbon • • • • • • Graphite fibres embedded in plastic material form high strength, lightweight composites. The composites are used in products such as tennis rackets, fishing rods, aircrafts and canoes. Being good conductor, graphite is used for electrodes in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalis. Being highly porous, activated charcoal is used in adsorbing poisonous gases Carbon black is used as black pigment in ink and as filler in automobile tires Coke is used as fuel and as reducing agent in metallurgy Important Compounds of C and Si: 1) Carbon Monoxide • • • • Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide Water gas (CO + H2) and producer gas (CO + N2) are very important industrial fuels 1σ and 2π bonds, 1 lone pair in each atom If present in blood, combines with haemoglobin to form a highly stable compound which reduces oxygen-carrying capacity of blood 2) Carbon Dioxide • • • • • • • • • Complete oxidation of C and C-containing fuels in the presence of air Colourless, odourless; low solubility makes it bio-chemically and geo-chemically important 0.03% of atmosphere H2CO3 is a weak dibasic acid Used in photosynthesis Increased CO2 in atmosphere causes global warming Dry ice is used as a refrigerant for ice-cream and frozen food CO2 is non-combustible, non-supporter of combustion so it is used as fire extinguisher Exists in 3 resonant structures with bond length=115pm 3) Silicon Dioxide • • • 95% of Earth’s crust is made of silica compounds Crystalline forms: Quartz, Cristobalite, Tridymite SO2 is a covalent 3D network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms 4) Silicones Group of organosilicon polymers with (-R2SiO-) groups repeating, where R is alkyl or aryl • Silicates are water-repelling • They have in general high thermal stability, high dielectric strength and resistance to oxidation and chemicals • They are used as sealant, greases, electrical insulators, for water proofing of fabrics, and in surgical and cosmetic plants. 5) Silicates • Eg: Feldspar, Zeolites, Mica, Asbestos • Si-atom is bonded to 4 in tetrahedron form • Silicate units are linked together in chain, ring, sheet or 3D forms • If all four corners are shared with other tetrahedral units, a 3D network is formed • Important man-made silicates: Glass, Concrete • 6) Zeolites • • • • If Al replaces few Si atoms, it is called Aluminosilicate, and acquires -ve charge Na+, K+, Ca2+ etc balance the -ve charge, and form zeolites/feldspar Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation Hydrated zeolites are used as ion exchangers in softening of hard water
