Uploaded by Sisir Lakkaraju

P-block Elements (G13, G14)

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P-block Elements
General Points:
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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:
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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:
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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:
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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:
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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:
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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:
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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:
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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
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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
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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
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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
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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
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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
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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
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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
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Silicates are water-repelling
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They have in general high thermal stability, high dielectric strength and resistance to
oxidation and chemicals
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They are used as sealant, greases, electrical insulators, for water proofing of fabrics, and in
surgical and cosmetic plants.
5) Silicates
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Eg: Feldspar, Zeolites, Mica, Asbestos
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Si-atom is bonded to 4 in tetrahedron form
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Silicate units are linked together in chain, ring, sheet or 3D forms
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If all four corners are shared with other tetrahedral units, a 3D network is formed
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Important man-made silicates: Glass, Concrete
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6) Zeolites
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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
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