atomic radii of some representative elements (in picometers)...borides - binary compounds with less...
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Atomic Radii of Some Representative Elements (in Picometers)
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• B, Al, Ga, In, and Tl; • Valence-shell electron configuration: ns2 np1
• Group 3A elements show increasing metallic character going down the group.
• Boron: a metalloid, forms covalent network solid, and highest melting point in the group
Group 3A, Group 13
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Some Physical Properties, Sources, and Methods of Preparation
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Some Important Reactions
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Properties of Boron (1s2 2s2 2p1)
• A metalloid; forms covalent network solid; highest melting point in the group, and the least reactive
• All boron compounds are covalent molecules; • Boron molecules, such as BF3, have
incomplete octet and acts as Lewis acid, example:
BF3 + :NH3 → F3B:NH3 • Boron hydrides acquire octet by forming H-
bridges, electron-deficit bonds; • Boron oxide, B2O3, forms weak boric acid,
B(OH)3; icosahedron
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Boranes - Wade rules
Lewis acids
€
3 LiAlH4 et( ) + 4 BF3 et( ) → 2 B2H6 g( ) + 3 LiAlF4 et( ) pyrolysis
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Borides - binary compounds with less electronegative elements - variable composition (M5B, M4B, M3B, M2B, MB, M7B3, M10B11, MB66
e.t.c.) - Very hard, chemically inert, high electric conductivity - Direct synthesis at high temperature:
MgB2 superconductor at 38 K
CaB6
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Selected applications
Boron nitride – BN isoelectronic with C – cubic lattice as diamond
Boron carbide – ceramics - approximately B4C
2 B2O3 + 7 C B4C + 6 CO
BN – nanotubes – isolectronic with C-nanotubes
Boronated metals – hard coating
Superhard materials:
Others:
Elemental boron fiber – light, high-strength - aerospace (composite) structures
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Aluminum
• Electron configuration: 1s2 2s2 2p6 3s2 3p1 or [Ar] 3s2 3p1
• Third most abundant element (and most abundant metal) in the Earth’s crust;
• Most important metal of Group 3A;
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Aluminum Production
• Extracted from bauxite, Al2O3⋅nH2O; • Produced by the Hall-Heroult process - electrolysis of
molten Al2O3-Na3AlF6 (cryolite) mixture at ~ 960oC • Aluminum production is an energy intensive process • Energy consumption: ~ 54 MJ/kg Al (~3% of
electrical energy supply) • Re-cycling saves up to 95% of this energy; • (Re-cycle an aluminum can and power your desk-top
monitor up to 3 hours)
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Importance of Aluminum
• Lightweight metal (density = 2.70 g/cm3); • Forms strong, lightweight alloys with copper and
magnesium for aircraft bodies and parts; • High resistance to corrosion - extensively used to
make beverage containers (soda drinks cans);
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Chemical Properties of Aluminum
• Reactive metal, readily oxidized by atmospheric O2 to form Al2O3;
• Al2O3 forms protective coating (an anodic protection) that prevents further corrosion of the metal;
• Al2O3 is amphoteric - reacts with both strong acids & bases;
• Aluminum reacts with halogens to form AlX3; • Al2O3 and AlF3 are strictly ionic compounds; • Other halides are ionic with covalent characteristics.
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Reactions of Aluminum
With strong acids: • 2Al(s) + 6 HCl(aq) → 2AlCl3(aq) + 3H2(g); • 2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g); • 2Al(s) + 2HNO3(aq) → Al2O3(s) + 2NO(g) + H2O(l)
(The third reaction does not occur completely because the oxide forms protective coating to prevent further reaction.)
With a strong base: • 2Al(s) + 6H2O(l) + 2NaOH(aq) → 2NaAl(OH)4(aq) + 3H2(aq);
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Important Compounds of Aluminum
• Al2O3 – source of aluminum metal and forms protective coating to the metal to prevent corrosion;
• Al2(SO4)3 – most important industrial compound; 1. use in municipal water treatment plants; 2. Prepared by reaction of H2SO4 with Al2O3 or Al(OH)3:
Al2O3(s) + 3H2SO4(aq) → Al2(SO4)3(s) + 3H2O(l) 2Al(OH)3(s) + 3H2SO4(aq) → Al2(SO4)3(s) + 6H2O(l)
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Exercise #5
1. Classify the following oxides as acidic, basic or amphoteric.
B2O3 Al2O3 In2O3
• Balance the following equations: 1. B(s) + HNO3(aq) → B2O3(s) + NO(g) + H2O 2. Al(s) + H2SO4(aq) → Al2(SO4)3(aq) + H2(g) 3. Al2O3(s) + HCl(aq) → AlCl3(aq) + H2O(l)
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Group 4A, Group 14 (vs: ns2 np2)
• Contains: a nonmetal (C), two metalloids (Si & Ge), and two metals (Sn & Pb);
• Carbon exists in 3 allotropic forms: graphite, diamond, and the “bucky-ball”.
• Graphite has sp2 hybridization; • Graphite is soft and conduct electric current; • Diamond contains sp3 hybridization and forms
covalent network solids; does not conduct electricity; • Diamond is the hardest material on Earth.
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Group 4A, Group 14
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Some Physical Properties, Sources, and Methods of Preparation
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Some Important Reactions
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Carbon: 1s2 2s2 2p2
• Most important element on Earth – forms the basic skeletal structures of all living things;
• Carbon forms strong covalent bonds with many elements and with itself;
• Carbon forms sp, sp2, and sp3 hybridizations; • In sp hybridization, each carbon forms 2 σ- and 2 π-
bonds; example in H―C≡C―H • In sp2, each carbon forms 3 σ- and a π- bonds; • In sp3 hybridization each carbon forms 4 σ-bonds;
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Important Compounds of Carbon
• CO – toxic gas (binds to hemoglobin); forms during combustion of carbon in limited oxygen supply; used in methanol production.
• CO2 – end-product of combustion of carbon or carbon-containing compounds; greenhouse gas that keeps Earth temperature relatively warm;
• CO2 is essential to life – used by plants in photosynthesis;
• NaHCO3 – used as baking soda for cooking and as in fire-extinguishers;
• Na2CO3 – used in glass manufacture; • CaCO3 – used in steel production;
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Methane clathrates
- CH4 in the ice crystal structure
- melting of 1 m3 clathrates releases 164 m3 of CH4
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Carbonyl halogenides - Planar – important for chemical synthesis - phosgene - Very toxic – chemical weapon
CO(g) + Cl2(g) Cl2CO(g)
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Other Important Compounds of Carbon
• Pseudohalogenides – derived from: HCN – hydrogen cyanide, HCNO – fulminic acid, HNCO – isocyanic acid,
HNCS – isothiocyanic acid and H2NCN – cyanamide.
Carbides: MxCy with M being less electronegative
- ionic: either as C–IV (carbides), (C2)2– (acetylides), (HC2)– (hydrogenacetylides)
- CaC2; CaO with carbon at 2200°C: - Covalent - Hard, chemically inert, high m.p. - SiC, B4C - Metalic Interstitial compounds – atoms of C occupy the octahedral
cavities in the crystal structures of metals (e.g. TiC, ZrC, VC, NbC, V2C, MoC, Mo2C, WC ...)
- high m.p. (3000-4500°C), hardness, electric conductivity - Transition carbides – between ionic and metalic carbides (e.g. Cr2C3,
Mn3C, Fe3C, Co3C ...)
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Chemistry of Silicon
• Silicon - a metalloid; a covalent network solid with diamond-like structure; very important in the electronic industries - forms semi-conductors.
• Silicon dioxide or silica (SiO2) - the second most abundant substance on the Earth’s crust; also the source of silicon – many polymorphs;
• SiO2 - used in the manufacture of glass and ceramics; • Silicon carbide (SiC) has diamond-like structure;
used to make abrasive and heat resistant ceramics.
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Production of Silicon
• SiO2(s) + 2C(s) → Si(s) + 2CO(g); • Si(s) + 2Cl2(g) → SiCl4(g); • SiCl4(g) + 2Mg(s) → Si(s) + 2MgCl2(s); • Final purification done by “zone-refining”
Silanes: structures similar to hydrocarbons, polymers
Siloxanes: polymer, silicon oil, rubber
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Silicon dioxide – many polymorphs
α-cristobalite tridimite β-quartz
melanophlogite stishovitee
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Silicates – some can be derived from H4SiO4
Island silicates
chain silicates
asbestos
Be3Al2Si6O18 - beryl α-Ca2SiO4
in cement
hemimorfit – Zn4(OH)2Si2O7
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Layer silicates and aluminosilicates
Clay minerals
caolinite
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3-dimensional aluminosilicates
Zeolites - absrobents, molecular sieve, ion-exchange Na2Al2Si3O10·2H2O
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• Germanium • Usually as admixtures in other minerals
and in some coal ashes • • transistors • Halogenides non-volatile
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Tin and Lead
• Both are soft metals; Tm(oC): Sn (232) & Pb (327) • Tin - used mainly in tin-plating for making food cans,
for making solders, bronze, and pewters; • Lead - mainly used to make automobile batteries;
some are used as lead shots and radiation shields; • Both metals form +2 and +4 oxidation states; • Reacts with O2 → SnO, SnO2, PbO & PbO2; • Reacts with Cl2 → SnCl2, SnCl4, PbCl2 & PbCl4; • SnO, PbO, SnCl2, and PbCl2 are ionic; • SnO2, PbO2, SnCl4, and PbCl4 are molecular;
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Important Compounds of Tin and Lead
• SnCl2 – used as reducing agent, tin plating, catalyst; • SnF2 – additive in toothpaste to prevent cavity; • PbO – used in ceramic glaze, and cement; • PbO2 – oxidizing agent and battery electrodes; • PbCrO4 – for making yellow pigment for paint;