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Covalent radii

The covalent radii increase down the group. The difference in size between Si and Ge is less than might be otherwise expected because Ge has a full 3d shell, which shields the nuclear charge rather ineffectively. In a similar way the small difference in size between Sn and Pb is because of the filling of the 4f shell.

Ionization energy

The ionization energy decreases from C to Si, but then changes in an irregular way because of the effects of filling of the d and f-shells. The amount of energy required to form M4+ ions is extremely large and hence simple ionic compounds are rare.

Melting points

Melting involves breaking the strong covalent bonds in the lattice of C and thus has extremely high melting point. The melting points decrease on descending the group.

Metallic and non-metallic character

The change from non-metal to metal with increasing atomic number is well illustrated in group IV, where C and Si are non-metals, Ge has some metallic properties, and Sn and Pb are metals.

Carbonates (CO32–) and bicarbonates (HCO3)

Carbonic acid is a dibasic acid and gives rise to two series of salts, carbonates (normal salts) and bicarbonates (acid salts), due to successive removal of the replaceable hydrogens from H2CO3.
Description: 51160.png
These can also be prepared using the following methods:
  1. With NaOH
    Description: 51167.png
    Description: 51175.png
  2. By precipitation
    Heavy metal caronbates are precipitated from their salt solutions with washing soda.
    Description: 51182.png
    While carbonates of many metals are known, bicarbonates of only alkali metals exist in the solid state.

Halides of carbon

Carbon combines with all the halogens to form tetrahalides, viz., CF4, CCl4, CBr4, and Cl4; mixed tetrahalides such as CFCl3, CF2Cl2, and CCl3Br; and trihalides of the formula CHX3, viz., CHCl3 (chloroform) and CHI3 (iodoform).
General characteristics
  1. The thermal stability of tetrahalides of carbon follows the order:
     CF4 > CCl4 > CBr4 > Cl4
  2. The tetrahalides, especially those containing both fluorine and chlorine, are chemically inert, non-inflammable gases or liquids.
  3. Freon (CF2Cl2) is used as a refrigerant.

Preparation of silicon and its reactions

SiO2 + 2Mg Si + 2MgO
The crystalline variety is obtained by heating finely powered sand or quartz with carbon in an electric furnace, a small amount of iron is added to prevent the formation of carborundum (SiC).
SiO2 + 2C Si + 2CO
It burns brilliantly in oxygen and ignites spontaneously in fluorine.
Si + O2 SiO2
Si + 2F2 SiF4
Si + 2KOH + H2O K2SiO3 + 2H2
Na2CO3 + Si Na2SiO3 + C
It combines with certain metals to form silicides.
2Mg + Si Mg2Si
When amorphous silicon is strongly heated, it fuses and on cooling solidifies to the crystalline form. It is very hard crystalline silicon, does not burn in oxygen, but readily combines with fluorine. It dissolves in mixture of HNO3 and HF. When fused with alkali, it gives a silicate.
Na2CO3 + Si Na2SiO3 + C

Compounds of silicon

  1. Silicones: These are organosilicon polymers containing Si–O–Si linkages. These are formed by the hydrolysis of alkyl or aryl substituted chlorosilanes and their subsequent polymerization. The alkyl or aryl substituted chlorosilanes are prepared by the reaction of Grignard reagent and silicon tetrachloride.
    Description: 51191.png
    Description: 51201.png
    Description: 51213.png
    Hydrolysis of substituted chlorosilanes yields corresponding silanols, which undergo polymerization. R3SiCl on hydrolysis forms only a dimer.
    R3Si OH + HOSiR3 R3Si – O – SiR3 + H2O
    1. The lower silicones are oily liquids but higher members containing long chains or ring structures are waxy and rubber-like solids.
    2. Silicones are stable towards heat.
    3. Chemical reagents have no action on silicones.
    4. These are non-toxic.
    5. Viscosity of silicone oils remains the same at different temperatures.
    6. Silicones are good electrical insulators.
    7. These are water repellents.
  2. Silicates: Silicates are metal derivatives of silicic acid, H4SiO4 or Si(OH)4. Silicates are formed by heating metal oxide or metal carbonates with sand, e.g.,
    Description: 51220.png
    Description: 52059.png
    Silicates have basic unit of Description: 51230.png, each silicon atom is bonded with four oxide ions tetrahedrally.
    There are following types of silicates:
    Orthosilicates: These silicates contain single discrete tetrahedral unit of Description: 51238.png.
    Pyrosilicates: These silicates contain two units of Description: 51249.png joined along a corner containing oxygen atom. These are also called as island silicate.
    Description: 52071.png
    Cyclic structure: Cyclic or ring silicates have general formula Description: 51259.pngDescription: 52408.png.
    Description: 52083.png
    Chain silicates
    • Simple chain silicates or pyroxenes are formed by sharing two oxygen atoms by each tetrahedral. Anions of such chain silicates have general formula Description: 51268.png.
      Description: 52100.png
    • Double chain silicates can be formed when two simple chains are joined together by shared oxygen atoms. These minerals are called amphiboles. The anions of such silicates have general formula Description: 51275.png.

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