By weight percentage (about 28%), silicon is second most common element in the earth's crest. It is found mainly as silicates. The most abundant is oxygen. It is the material used to make computer chips. Germanium, tin and lead are not so abundant, but are also importance commercial elements. The Latin word for tin is stannum, and for lead is plumbum. Before plastic was known a plumber is a person who works on lead pipes.
CRYSTALLINE FORMS OF THE ELEMENTS
The crystals of silicon and germanium have the diamond form. The graphite form does not exist because the p-orbitals are unable to interact to form π-bonds.
For tin the diamond form (or α-tin or grey tin) exist only below 13�C. At a higher temperature, up till 161�C, it changes into the β-form (or white tin), and above 161�C into the γ-form. Tin melts at 232�C.
REACTIVITY OF THE ELEMENTS
This Group is right at the center in the Periodic Table. So carbon reacts strictly via the sharing of valence electrons.
Silicon showed very slight ionic tendencies in SiH4. The other elements show even a greater tendency to give away electrons. Silicon is not attack by acids. It is attack by halogens to give tetrahalides and by alkalies to give silicates. Germanium is slightly more reactive. It would dissolve in concentrated sulphuric acid and nitric acid. Tin and lead are even more reactive.
Note: Hydrofluoric acid (HF) can attack almost all elements including silicon
THE HYDRIDES
The hydrides of silicon are known as silanes. The simplest member is SiH4. However unlike methane the Si-H bond is slightly ionic (Si+H‾). Silanes are flammable in air. It is easily oxidised by oxygen to give the dioxide.
It reacts explosive with halogens at room temperature. They are stable to hydrolysis below the pH of 7. Above this pH it is easily hydrolysed to silica.
Silicons form weak Si-Si bonds producing a great variety of silanes. Si2H6 are stable at room temperature. The higher silanes decompose on standing. The largest silane prepared is Si6H14.
The hydrides of germanium are known as germanes. Germanes are less inflammable than silanes in air and also more stable to hydrolysis. However they are still readily oxidised by oxygen. The highest member prepared was Ge9H20.
For tin and lead only SnH4, Sn2H6, and PbH4 and known.
Tutorial 1
What names would you propose for the hydrides of tin and lead? Answer
OXIDES
Silicon dioxide is known as silica. Although it can exist in several allotrophic forms, the stable form is quartz, commonly found in nature. The other forms are metastable. Silica in its natural form is resistant to the halogens and acids, however it will react with alkalies to give silicates.
Silica can also fused with alkali carbonates at 1800�C to give the silicate.
        Si(O)3-O-Si(O)3 |
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Silicon oxo anions are found as silicates in the soil and rocks. However few of them are present as a simple discrete tetrahedral orthosilicate anion, SiO4‾4. The silicate units are more often bonded together via the oxygen atom. A simple example would be [3OSi-O-SiO3]‾6. Note that for the 2-D representation the three bonds are actually radiating from the silicon atom. The centre oxygen is above this silicon atom.
Of course more than two units can be bonded to form larger linear chains. These chains can also bond to each other to form sheets of silicates. If the metal cation-to-silicate ratio and the chemistry are correct the silicate anions can form cyclic units.
   
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    Si6O18‾�� |
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So far we have been assuming that the oxygen above the silicon is bonded to a metallic cation. However if the cation is divalent (two positive charge) the sheets can then be bonded to each.
This ability of the silicate to accommodate all sorts of chemical formulation makes it a very interesting commercial material.
Of course the sheet need not spread out in one direction. Molecules seldom behave in this manner since the bonds are seldom 180�C. In this case the tetrahedral nature of the bonds will curl the molecules and very often the terminal units can bond together to form a "ball" molecule, known as zeolites. The metals attached onto the oxygen anion ranges from sodium, potassium, calcium, magnesium and aluminum. They are now very important commercial materials; as catalysts, molecular sieves, and cation exchangers. The preparation of tailored zeolite is now an important area of chemistry.
This is where I advice you to try to construct the molecules in three dimension using your creativity. However you can purchase a set of atoms and bonds to make this simple. A very simple set that do not cost too much will do.
The oxides GeO2, SnO2, and PbO2 are of lesser importance. SiO2 is acidic and GeO2 to a lesser degree. SnO2 is amphoteric, while PbO2 is rather inert.
Germanates, stannates and plumbites are also known. Germanates do show some of the chemistry of silicates.
SALTS
All the halides are known, except for PbBr4 and PbI4.
The silicon halides, except SiF4, are easily hydrolysed by water to give silicic acid. The halides of the other elements are similarly hydrolysed but not to the extent shown by silicon halides.
The Group IVB elements form few salts of oxo acids. Sn(SO4)2 and Sn(NO3)4 are known.
LOWER VALENCE COMPOUNDS
As the elements get larger, we will have to consider the d- and f-electron shells. These would make the elements to deviate from following the "octet rule". It is now possible for germanium, tin and lead to give away just two of the valence electrons to form GeCl2, SnCl2 and PbCl2. We say that they are in a divalent state. Such compounds are not very stable. Germanium dichloride would decompose when left standing at 75�C;
In chemistry when the molecules "react with itself" to give two different products we referred to it as a disproportionation reaction.
SnCl2 is known as stannous chloride. If it is in the tetravalent state they are known as stannic salts. Example; SnCl4 is known as stannic chloride. A similar system is used for lead. PbCl2 is known as plumbous chloride and PbCl4 is known as plumbic chloride.Stannous chloride dissolves readily in non-aqueous solvent that has a lone electron pair (oxygen or nitrogen atoms). In water it is would be oxidised to the stannic cation. The oxide SnO is metastable, and the phosphite, SnHPO3, is a stable crystal.
The plumbous salts are even more stable. We can have Pb(NO3)2, PbSO4, PbCrO4, PbO, PbS, besides the chloride.
