Group IV Elements 42.1 42.2 42.3 42.4 1 Introduction Characteristic Properties of the Group IV Elements Composition of Chlorides and Oxides of the Group IV Elements Silicon and Silicates Introduction The Group IV elements 2 carbon silicon germanium tin lead exhibit a marked change (dissimilarity) among the elements in the same group Introduction Carbon dull black in the form of graphite 3 Introduction Carbon hard and transparent in the form of diamond 4 Introduction Silicon and germanium dull grey or black Si 5 Ge Introduction Tin and lead shiny grey Sn 6 Pb Introduction The Group IV elements outermost shell electronic configuration of ns2np2 Element 7 Electronic configuration Carbon [He] 2s22p2 2,4 Silicon [Ne] 3s23p2 2,8,4 Germanium [Ar] 3d104s24p2 2,8,18,4 Tin [Kr] 4d 105s25p2 2,8,18,18,4 [Xe] 4f 145d 106s26p2 2,8,18,32,18,4 Lead Structure and Bonding Moving down the group carbon non-metal silicon germanium tin lead 8 metalloids metals Structure and Bonding Most common structure : giant covalent structure Examples carbon silicon germanium grey tin (an allotrope of tin) 9 Allotropes are different structures of the same element 1. Carbon 10 two important allotropic forms diamond and graphite Structures of diamond 11 each carbon atom is bonded to four other C atoms 12 extremely hard and chemically inert 13 All electrons are localized non-conductor 14 Structure of graphite 15 Graphite layered structure Covalent bonds van der Waals’ forces 16 The layers slide over each other easily 17 brittle and soft Electrons between layers are delocalized conducts electricity along the layers 18 2. Silicon and Germanium 19 network lattice the atoms are covalently bonded to one another 3. Tin and Lead Tin two allotropes white tin and grey tin White tin 20 cold heat Grey tin 3. Tin and Lead White tin 21 stable form metallic lattice structure atoms are held together by metallic bonding conducts electricity shows the properties of a typical metal 3. Tin and Lead Grey tin network lattice structure similar to that of diamond White tin more dense cold heat Grey tin less dense White tin expands and crumbles on cooling Napoleon’s retreat from Russia 22 3. Tin and Lead Lead 23 typical metallic lattice atoms are held together by metallic bonding Some physical properties of the Group IV elements C Si Ge Electronegativity value 2.5 1.74 2.0 Electronic configuration 1s22s22p2 [Ne] 3s23p2 [Ar] 3d104s24p2 0.077 0.117 0.122 347 226 188 Melting point (C) 3527 1414 1211 Boiling point (C) 4027 3265 2833 716 456 376 Atomic radius (nm) Bond enthalpy (kJ mol–1) Enthalpy change of atomization (kJ mol–1) 24 Some physical properties of the Group IV elements Tin (Sn) Lead (Pb) Electronegativity value 1.7 1.55 Electronic configuration [Kr]4d10 5s25p2 [Xe] 4f145d10 6s26p2 0.140 0.154 Bond enthalpy (kJ mol–1) 150 – Melting point (C) 232 327 Boiling point (C) 2602 1749 302 195 Atomic radius (nm) Enthalpy change of atomization (kJ mol–1) 25 Variation in Melting Point on going down the group 26 C 3527C Si 1414C Ge 1211C Sn 232C Pb 327C Variation in Melting Point The very high m.p. of diamond is due to the strong C – C bonds & the giant structure C 3527C Si 1414C Going from C to Ge Ge 1211C bond length Sn 232C bond strength Pb 327C melting point 27 Variation in Melting Point 28 Sn and Pb have exceptionally low m.p. because C 3527C Si 1414C 1. metallic structures Ge 1211C extent of bond breaking on melting is small Sn 232C Pb 327C Variation in Melting Point Sn and Pb have exceptionally low m.p. because 2. only two (ns2) of the four valence electrons are involved in the sea of electrons Tin (Sn) Lead (Pb) [Kr]4d10 5s25p2 [Xe] 4f145d10 6s26p2 29 C 3527C Si 1414C Ge 1211C Sn 232C Pb 327C Variation in Boiling Point The general trend and explanation similar to those for m.p. C 4027C Si 3265C Ge 2833C 30 Sn 2602C Pb 1749C Chlorides Two series of chlorides formed by the Group IV elements 31 the dichlorides (MCl2) the tetrachlorides (MCl4) Chlorides All Group IV elements form tetrachlorides liquids at room temperature and pressure GeCl4 all are simple covalent molecules with a tetrahedral shape PbCl4 32 CCl4 SiCl4 SnCl4 - + - - M – Cl bonds are polar with ionic character Molecules as a whole are non-polar 33 Reactions with water CCl4 + H2O no reaction SiCl4 + H2O Si(OH)Cl3 + HCl 34 H Cl + Si Cl Cl O Cl Cl Cl H Si Cl H O Cl H Si in SiCl4 is more positively charged than C in CCl4 More susceptible to nucleophilic attack Si, unlike C, can expand its octet to accept an additional electron pair 35 H Cl + Si Cl Cl O Cl Cl Cl Si Cl H O H Cl H Cl Si Cl Cl OH H4SiO4, silicic acid 36 + HCl Reactions with water CCl4 + H2O no reaction SiCl4 + H2O Si(OH)Cl3 + HCl Si(OH)Cl3 + H2O Si(OH)2Cl2 + HCl Si(OH)2Cl2 + H2O Si(OH)3Cl + HCl Si(OH)3Cl + H2O Si(OH)4 + HCl H4SiO4, silicic acid 37 Chlorides tendency to form dichlorides, MCl2 down the group 38 all possess covalent character though they exist as crystalline solids at room temperature and pressure GeCl2 SnCl2 PbCl2 Chlorides On moving down the group, Metallic character of elements Ionic character of MCl2 mainly covalent mainly ionic 39 GeCl2 SnCl2 PbCl2 Chlorides On moving down the group, the relative stability of +4 oxidation state the relative stability of +2 oxidation state 40 Tin (Sn) Lead (Pb) [Kr]4d10 5s25p2 [Xe] 4f145d10 6s26p2 The outermost ns2 electrons are less shielded by the more diffused inner d and/or f electrons. They are attracted more by the positive nucleus Less available for forming bonds Form only two bonds using np2 41 Oxides Two series of oxides formed by the Group IV elements 42 the monoxides (MO) the dioxides (MO2) Oxides All Group IV elements form the dioxides Carbon dioxide 43 the only Group IV dioxide which consists of simple molecules exists as a gas at room temperature and pressure Oxides The dioxides of other Group IV elements 44 crystalline solids of high melting points either giant covalent or giant ionic structures Oxides All Group IV elements (except silicon) form the monoxides at normal conditions Stability of MO down the group CO GeO SnO PbO 45 Oxides CO2 SiO2 GeO2 SnO2 PbO2 46 Decreasing stability of dioxide Tin (Sn) Lead (Pb) [Kr]4d10 5s25p2 [Xe] 4f145d10 6s26p2 The outermost ns2 electrons are less shielded by the more diffused inner d and/or f electrons. They are attracted more by the positive nucleus Less available for forming bonds Form only two bonds using np2 47 The bond type and the relative stabilitiy of the monoxides and dioxides formed by the Group IV elements Group IV element Carbon Silicon Oxides formed Bond type of the oxide Relative stability CO Covalent Unstable (reducing) CO2 Covalent Stable (SiO) – Very unstable SiO2 Covalent Stable GeO Predominantly ionic Unstable in the presence of O2 GeO2 Partly ionic, partly covalent Stable Germanium 48 The bond type and the relative stabilitiy of the monoxides and dioxides formed by the Group IV elements Group IV element Oxides formed SnO Tin SnO2 PbO Lead 49 PbO2 Bond type of the oxide Relative stability Predominantly ionic Unstable (reducing) Partly ionic, partly covalent Unstable (oxidizing) Ionic Stable Predominantly ionic Unstable (oxidizing) Silicon and Silicates Silicon 50 the second most abundant element in the Earth’s crust about 28% by mass commonly found as silicon oxide (also known as silica) oxygen 46.6% silicon 27.7% aluminium iron 8.1% 5.0% all other elements 1.5% magnesium 2.1% 51 calcium 3.6% sodium 2.8% potassium 2.6% Percentage abundance by weight of major elements in the Earth’s crust. Silicon Example: 52 in a variety of forms such as sand, quartz and flint also found as silicates in rocks and clay Preparation of Silicon 1. by reduction of silica with carbon in an electric furnace SiO2(s) + 2C(s) Si(s) + 2CO(g) 2. Extremely pure silicon can be obtained by the reaction of silicon(IV) chloride with hydrogen SiCl4(s) + 2H2(g) Si(s) + 4HCl(l) followed by zone refining of the resultant silicon 53 Applications of Silicon Silicon is the basic material 54 for making semi-conductors used in the construction of transistors and rectifiers for making steel and aluminium alloys Chemistry of Silicon dominated by its strong tendency to form Si–O single bond reflected by its formation of silica and a variety of silicates Silicates consist of Si, O and one or more metals 55 Structures and Bonding of Silicates 1. SiO44– as the Basic Chemical Unit of Silicates Covalent bond one electron (symbol ‘O’) of each oxygen atom is gained from another atom (usually metal atom) 56 4- O 109.5º Si O O O SiO44 57 For simplicity, a SiO44– tetrahedron can be represented by a pyramid as follows: or Si > O not drawn to scale 58 SiO44- tetrahedron apex towards us Types of silicates • • • • • Nesosilicates (lone tetrahedron) - [SiO4]4−, eg olivine. Sorosilicates (double tetrahedra) - [Si2O7]6−, eg epidote. Cyclosilicates (rings) - [SinO3n]2n−, eg tourmaline group. Inosilicates (single chain) - [SinO3n]2n−, eg pyroxene group. Inosilicates(double chain) - [Si4nO11n]6n−, eg amphibole group. (not required in A-Level) • Phyllosilicates (sheets) - [Si2nO5n]2n−, eg micas and clays. • Tectosilicates (3D framework) - [AlxSiyO2(x+y)]x−, eg quartz, feldspars. http://www.windows.ucar.edu/tour/link=/earth/geology/silicates2.html 59 Different structures of silicates 1. Isolated silicates Contain isolated SiO44– tetrahedra. or 60 1. Isolated silicates are not polymerized. are bonded to the metal ions (e.g. Mg2+ or Fe2+) by ionic bonds. tend to have high densities and are not easy to cleave. 61 Olivine ((Mg,Fe)2SiO4) is an example of isolated silicates. It is the most abundant mineral in the Earth’s mantle. 62 Deducing the chemical formula of an isolated silicate, (Fe,Mg)2SiO4 The diagram below shows an incomplete part of an isolated silicate. This part, when complete, can repeat infinitely to give a three-dimensional structure of the silicate. silicate tetrahedron magnesium ion 63 (a) Add an appropriate number of (representing iron(II) ion) in the figure above to make up a complete part of the silicate. Charge on each Mg2+ ion is +2 Charge on each SiO4 4– ion is –4, the net charge of the incomplete part = (+2)(8) + (–4)(8) = –16 To balance the charge, the number of Fe2+ ions needed = 8 64 For regular packing of particles, one of the feasible way of putting Fe2+ ions is shown below: (b) Hence, deduce the chemical formula of the silicate. From the picture, the formula of the part is Fe8Mg8(SiO4)8 FeMgSiO4. 65 M1 = Mg2+; M2 = Fe2+ No. of Fe2+ = 4 Fe4Mg4(SiO4)4 FeMgSiO4. No. of Mg2+ = 4¼ + 4½ + 1 = 4 66 1. SiO44– as the Basic Chemical Unit of Silicates • Zircon (ZrSiO4) an example of isolated silicate mineral the principal ore of zirconium metal 67 1. SiO44– as the Basic Chemical Unit of Silicates • Zircon (ZrSiO4) brilliant appearance high refractive index used as a diamondlike gem 68 Double tetrahedra – Sorosilicates The Si2O76– anion is formed by joining two SiO44– tetrahedra together through a common oxygen atom The negative charges are present only on the oxygen atoms that are not shared by the two silicon atoms 69 Double tetrahedra – Sorosilicates 70 Double tetrahedra – Sorosilicates 71 2. Structures of Silicates • The SiO44– tetrahedra can be joined up by sharing oxygen atoms form ring, chain, sheet or network silicates 72 Cyclosilicates (rings) - [SiO3]n2n− O shared by two Si [SiO3]626 [Si6O18]12 Carry one –ve charge 73 Cyclosilicates (rings) - [SiO3]n2n− [SiO3]626 [Si6O18]12 74 Single chain silicates Each SiO44– tetrahedron possesses 1 Si atom and 3 (i.e. 1 + 1 + 0.5 + 0.5) O atoms. The ratio of Si to O is 1 : 3. 75 Stoichiometry : (SiO3)n2n– Single chain silicates 76 (a) Draw a diagram to show that when two oxygen atoms of each SiO44– tetrahedron are used for sharing, an infinite polymer chain can be formed. Your diagram should have four SiO4 4– tetrahedra. 77 O Si O O O SiO44– tetrahedron (b) Hence, show that the general formula of the silicate is (SiO3)n2n–. Key: oxygen silicon Number of Si atoms = 4 Number of O atoms = 11 (inside the chain) 0.5 × 2 (ends of chain) = 12 Charge on oxide = 8 (bridging O has no charge) the formula is Si4O128 the general formula is (SiO3)48 or (SiO3)n2n 78 Single chain silicates - pyroxenes Generally dark-coloured Commonly found in igneous rocks 79 Pyroxene minerals 2+ (SiO3)24 2+ • Schefferite, Ca(Mg,Fe,Mn)Si2O6 2+ 2+ • Zinc schefferite, Ca(Mg,Mn,Zn)Si2O6 2+ 2+ • Jeffersonite, Ca(Mg,Fe,Mn,Zn)Si2O6 3+ • Leucaugite, Ca(Mg,Fe,Al)(Al,Si)2O6 2+3+ • Calcium-Tschermak's molecule, CaAlAlSiO6 Half of Si atoms replaced by Al 80 (AlO3)3(SiO3)2 Double chain silicates Every other tetrahedron in a single chain shares a third oxygen atom with a neighbouring chain. 81 Double chain silicates In every two SiO44– tetrahedra, No. of Si = 2 Si : O = 2 : 5.5 = 4 : 11 Stoichiometry : Si4O116– No. of O = (1 + 1 + 0.5 + 0.5) + (1 + 0.5 + 0.5 + 0.5) = 5.5 82 1st tetrahedron 2nd tetrahedron +27 -62 -12 Amphibole (Ca2Mg5(Si4O11)2(OH)2) is an example of double chain silicates. 83 It is more difficult to cleave across the chains For both single chain silicates and double chain silicates, the Si–O covalent bonds within the chain silicate crystals are strong. 84 It is easier to cleave along the chain direction The ionic bonds between the metal ions and polymeric silicate chains are comparatively weaker than the Si–O covalent bonds. 85 Also, cleave between the front and the back chains Two planes of cleavage are perpendicular to each other 86 Chain silicate anions are found in fibrous asbestos 87 Single chain silicates Glass : CaSiO3 long chain anions can be drawn into long wire or glass wool. Limestone : CaCO3 Individual anions hard and brittle 88 Carbonate ions do not exist as long chains. Why? Carbon can form strong bond with oxygen. Moreover, carbonate ions can be stabilized by delocalization of electrons. - 89 O O O- O- C C C O- O O- - O O Also, polymerization of CO32 ions gives polymeric (CO2)2 which is electrically neutral O O O C C C O O O Stability : CO2 > (CO2)n O 90 C O O Sheet silicates Each SiO44– tetrahedron shares three oxygen atoms with neighbouring SiO44– tetrahedra 91 Each tetrahedron has 1 Si atom and 2.5 O atoms (1+0.5+0.5+0.5) 92 carries one negative charge (SiO2.5)nn or (Si2O5)n2n or (Si4O10)n4n 93 94 Structure of sheet silicates 4 1 Talc (Mg3(Si4O10)(OH)2) is an example of sheet silicates. It is an important filler material for paints, rubber and insecticides. Talc(滑石) 95 van der Waals’ forces a sandwichlike layer with Mg2+ and OH trapped in the middle Only weak van der Waals’ forces exist between the sheets of the SiO44– anions only one plane of cleavage 96 Si O OH Mg2+ Since talc is a relatively soft silicate, it can be pulverized to make talcum powder. It is a soft and fine powder used after bath to make our body feel smooth and dry. It is slippery because the layers can slide over one another 97 Sheet silicates Mica(雲母) Clay(黏土) Mica and clay readily cleave into thin slices (only one plane of cleavage) 98 Sheet silicates Mica(雲母) Clay(黏土) Mica is used as window glass and waveguide cover in microwave oven 99 4. Network silicates Consist of SiO44– tetrahedra in which ALL oxygen atoms are shared with adjacent SiO44– tetrahedra Electrically neutral Each tetrahedron has one Si atom and 2 O atoms (40.5) Si : O = 1 : 2 SiO2 (Quartz) 100 4. Network silicates each Si atom is attached to 4 O atoms each O atom is attached to 2 Si atoms Si SiO2 O No plane of cleavage much harder than other types of silicates 101 very high melting points 4. Network silicates Examples:- 102 Quartz Feldspar(長石) 4. Network silicates Feldspar - the most abundant group of minerals in the Earth’s crust - every oxygen atom is shared between SiO44– tetrahedra - some Si atoms are replaced by Al atoms Formula of anions : [(AlO2)x(SiO2)y]x− or [AlxSiyO2(x+y)]x 103 4. Network silicates Feldspar Formula of anions : [(AlO2)x(SiO2)y]x− or [AlxSiyO2(x+y)]x when x=0 SiO2 104 Types of Feldspar K-feldspar endmember KAlSi3O8 K+[(AlO2)(SiO2)3] Albite endmember NaAlSi3O8 Na+[(AlO2)(SiO2)3] Anorthite endmember CaAl2Si2O8 Ca2+[(AlO2)2(SiO2)2]2 105 Comparison of different types of silicates Structure Number of O atoms Formula of shared by each Si : O anion tetrahedron Isolated silicate 0 SiO44– 1:4 Single chain silicate 2 SiO32– or 1:3 Si2O64– Double chain silicate 106 2.5 Si4O116– 4:11 Number of O atoms Structure Sheet silicate shared by each tetrahedron 3 Formula of anion Si2O52- or Si : O 2:5 Si4O104- Network silicate 4 SiO2 or [(AlO2)x(SiO2)y]x 107 1:2 Structure Isolated silicate Properties • high density • not easy to cleave in a particular direction Single chain silicate • not easy to cleave the crystal across the chains Example Olivine, (Mg,Fe)2SiO4 Pyroxene, CaMg(Si2O6) • Two planes of cleavage Double chain silicate • not easy to cleave the crystal across the chains • Two planes of cleavage 108 Amphibole, Ca2Mg5(Si4O11)2(OH)2 Structure Sheet silicate Properties • soft • easy to be pulverized to make powder • cleavage usually occurs between sheets (only one plane of cleavage) Network silicate Example •very hard •high melting point Talc, Mg3(Si4O10)(OH)2 Quartz, SiO2 Feldspar, Ca(Al2Si2O8) 109 Predict the type of each silicate from its chemical formula. (a) CaMg(SiO3)2 single chain (b) Fe3Al2Si3O12 isolated (c) Si12Mg8O30(OH)4 sheet (d) Na2Mg3Al2Si8O22(OH)2 double chain (e) Mg(Al2Si2O8) network (feldspar) (f) MgAl(AlSiO6) single chain 110