GROUP IV ELEMENTS Group IV consists of Carbon – C, Silicon – Si, Germanium – Ge, Tin – Sn and Lead – Pb. This is one of the groups in which the transition from non-metals, C and Si through typical metalloid Ge, to weakly electropositive metals, Sn and Pb can be clearly seen. Valence shell – ns2np2 Table 1: Physical properties of Group IV elements Property C(6) Si(14) Ge(32) Sn(50) Pb(82) Electronic [He]2s22p2 [Ne]3s23p2 [Ar]3d104s24p2 [Kr]4d105s25p2 [Xe]4f145d106s26p2 configuration Electronegativity 2.5 1.74 2.0 1.7 1.55 IE(kJmol-1) 1st 1086 787 760 707 715 2nd 2352 1577 1534 1409 1447 3rd 4619 3230 3308 2943 3087 4th 6220 4355 4409 3821 4081 Melting pt (oC) 3550(diamond) 1410 937 232 328 Boiling pt (oC) 4827 2355 2830 2260 1744 Density (g cm-3) 3.51(diamond) 2.32 5.38 7.31 11.48 2.22(graphite) Occurrence Among all the elements of this group, carbon is the only one to occur in the elemental state as diamond and graphite. Silicon is the second most abundant element in the earth’s crust forming about 27.7%. Silicates are present in the rocks. Clays are essentially alumino-silicates of Na and Ca. Common sand is an impure form of silica. Glass is also a mixture of silicates. Germanium is a rare element. It occurs in traces in coal, in rare mineral argyrodite, 4Ag2S.GeS2, in germinate, Cu3(Ge.Fe)S4, and as admixture in zinc and tin ores. Tin occurs mainly as cassiterite or tin stone,SnO2 and Lead as galena, PbS. General characteristic and Important features of the Group 1. Carbon and silicon are outstanding but different. Carbon forms so many organic compounds; its chemistry is based on chains and rings of carbon atoms. Silicon forms many silicate rocks and its chemistry is dominated by polymeric silicate anions. This is due to their medium atomic size and their atomic configuration, ns2np2. In sp3 hybridization, all the four electrons become equivalent. Here their coordination number (4) is equal to the valence which gives the number of valence electrons which an atom uses in bonding. Thus, the resulting tetrahedral structures are also very symmetrical in their charge distribution since carbon and silicon have also medium electronegativities. 2. The properties of Ge, Sn and Pb change gradually as the group is descended, the metallic character increases down the group. 1 3. Oxidation state or valence They readily form tetravalent derivatives. However they show oxidn states of +2 and +4 which are also the valencies shown by these elements, except carbon where oxidation state varies from -4 to +4. 4+ All compound of +4 oxdn state are covalent because the IE to form M is too high to allow the formation of such ions. The heavier elements readily form lower oxidn state of +2; which become more ionic going down the group. Tin(II) and its compounds are often ionic. Only carbon forms anions due to its 2- 4- relatively high electronegativity, e.g., C2 in the carbides (acetylides) and C in methanides. 4. Coordination number: The maximum coordination number of carbon is four because it can never accommodate more than 8 electrons in its valence shell. Other members can expand the octet due to their d- orbitals. Thus, they can have higher coordination numbers and 2- 2- form complex ions e.g., SiF6 , PbCl4 etc. 5. Catenation: Is a property by virtue of which elements form long chins or rings compounds by single or multiple bond formation between atoms of the same element. The tendency to catenate decreases going down the group C, Si, Ge, Sn, Pb i.e., C> Si> Ge~ Sn> Pb catenation decrease Carbon shows this tendency to a remarkable extent because of its small size and high bond energy. Si and Ge to a lesser extent and Sn and Pb to a very small extent. E.g. Hydrides and halides Carbon: there are alkanes e.g. ethane, propane, butane, etc.of the formular CnH2n+2 n up to n= 100 are known. There are also others like alkenes and alkynes etc. Carbon also forms a number of catenated halides e.g., Polyvinylchlorides (PVC). Their formular is Cl (CH CH-) 2 n Silicon: forms silanes having the general formular SinH2n+2 where n≤ 8 Its halides also catenation and have a general formular SinX2n+2 Where if X=F, n= 1- 14 X = Cl, Br or I, n = 1 – 10 Germanium: forms germanes GenH2n+2, where n = 1 – 9 2 Halides form dimmers e.g., Ge2Cl6 Tin and Lead: while SnH4 & Sn2H6 have been prepared, preparation of PbH4 has failed. No catenated halides are known for Sn and Pb (6) Allotropy: The existence of elements in two or more different forms. These forms are called allotropes and the phenomenon is called allotropy. Allotropy occurs because of different crystal structures and shapes in the solid state and/ or combination of different numbers of atoms into molecules of simple compounds. Carbon: Has two forms (allotropes) namely diamond and graphite. They differ in their physical and chemical properties because of differences in their arrangement and bonding of the atoms i.e. they have different structures. They are, infact, macromolecules consisting of C atoms linked by a network of covalent bonds. Diamond: C 1.54Ao C C C C • Diamond structure consisting of carbon atoms surrounded by a tetrahedral arrangement of other carbon atoms at a distance of 1.54Ao. i.e. each carbon atom is tetrahedrally surrounded by four other carbon atoms which are covalently bonded to it. E(C-C) bond energy = 355kJmol-1 • Each C atom displays sp3 hybridization. • Since the structure possesses no free (mobile) electrons i.e. all valencies are satisfied (saturated); it behaves as an insulator and it is inert. • The rigid, strong three dimensional linkages make diamond one of the hardest substances known. • Since the covalent bonds must be broken before melting and boiling, it has an extremely high m.pt (3550oC) and b.pt (4827oC). 3 Graphite: 1.42Å First layer 3.35Å Second layer • Graphite has a two dimensional layer structure. Its lattice consists of plane layers or sheets in which carbon atoms are arranged in regular planar hexagons. • Carbon atoms are sp2 hybridized. Here each carbon atom is linked to other three carbon atoms through 3 bonds and one electron which has a overlap. This electron is delocalized over the whole planar layer. • These delocalized electrons make graphite a good conductor of electricity within the layer. Metallic lustre can also be attributed to these delocalized electrons. • Different layers are at a distance of 3.35Ao from each other and are thus held by weak Van der Waals forces only. These layers can slide over one another easily, imparting softness and slippery touch to graphite. Hence it is used as a lubricant and in pencil. • Due to its giant layer structure, it is very heat resistant and hence used for crucibles. • It is chemically very inert, thus, it is employed as an electrode material in many electrolyses. Silicon, Germanium & Lead: They exhibit only one allotropic form. Si and Ge crystallize in the diamond structure while Pb exhibit cubic closest- packed structure. Tin: Shows three modifications below its m.pt 232oC. They are interconvertible as shown in the following equilibrium: 4 – Sn – Sn – Sn 132oC 161oC HYDRIDES All the elements in group IV form covalent hydrides. The stability of the hydrides decreases down the group because of the decreasing M–H bond energies. Thus, while CH4 and other alkanes are stable compounds, PbH4 has not even been properly characterised, so far. Carbon: Why there are so many hydrocarbons in C? Because carbon undergoes catenation than any other element for both single and multiple bonds i.e., C – C, C = C and C C. This property is related to the bond strength E.g.: C-C = 346 kJmo-1 strong Si-Si = 226 kJmol-1 Ge-Ge = 186kJmol-1 -1 Sn-Sn = 151kJmol weak Catenated Pb compounds are unknown. The C–C bond strength is due to perfect overlap of the 2p orbitals which are also small in size hence intensive interaction. As the group is descended i.e. from Si to Pb the elements get larger and they possess d- orbitals. Thus, electrons partly diffuse into these vacant d- orbitals hence weak overlap of p orbitals. Si Ge Sn Pb further weaker Silicon, Tin and Lead: There are no silicon analogues of alkenes and alkynes (e.g. C2H4, C2H2). This is because poor overlap results from interaction of a 3p orbital of an atom of a 3rd period element and a 2p orbital of 2nd period or of two 3p orbitals of two atoms of the 3rd period elements. SinH2n+2 (n≤6) and GenH2n+2 (n≤6) are known. SnH4, Sn2H6 and PbH4 decompose slowly at room temperature and quickly when heated. 5 HALIDES All the elements of the group form dihalides (except carbon) and tetrahalides. All the four tetrahalides, namely, fluorides, chlorides, bromides and iodides are known for all the elements of the group, except PbBr4 and PbI4. Due to inert pair effect the stable oxidation state of Pb is +2. Therefore Sn and Pb form all the dihalides. Points to note: 2- • C–F, forms no complexes analogous to fluorosilicates SiF6 ions. This is attributed to the absence of d orbitals in the valence of carbon atom. Others have d orbitals available for coordination. • There is steady decrease in the C–X bond energies i.e., C-F>C-Cl>C-Br>C-I.