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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Silicon Page 1

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1.

Position of Silicon in the Periodic Table

 

  • Silicon belongs to Group IVA of the long form of the periodic table. The symbol of silicon is Si, atomic number is 14 while atomic weight is 28.
    The electronic configuration of silicon is 1s2, 2s2, 2p6, 3s2 3p2. Silicon belongs to the third period of the periodic table because its third shell is incomplete.
  • The other elements belonging to Group IVA are carbon (C), germanium (Ge), tin (Sn) and lead (Pb).
  • The electronic configuration of the outermost shell of all the elements of Group IVA is ns2, np2. Hence, they all are tetravalent.
  • The last electron of all the elements of Group IVA enters the 'p' - orbital. Hence, all these elements are classified as p-block elements.
  • The Group IVA elements are placed between the electropositive metals on the left hand side and electronegative non-metallic elements on the right hand side.

 

 

2.

 Group Relationship

 

The elements of Group IVA show the following characteristics:

  • In the group, the atomic number increases from carbon to lead.
  • With increase in atomic number, atomic radii increases and metallic character increases from carbon to lead.
  • The ionisation, potential, electronegativity, melting and boiling points decrease from carbon to lead.
  • The elements of Group IVA exhibit + 2, + 4 and - 4 oxidation state. The + 4 oxidation state is exhibited by forming covalent bonds with strongly electronegative elements like halogens or oxygen. The - 4 oxidation state is exhibited with highly electropositive elements.
  • The divalent compounds of lead show more stability than the quadrivalent compounds as the 6s2 electrons remain inert. This is known as the inert pair effect.

 

 

3.

Occurrence

 

In the combined form, silicon occurs mainly in two forms: (1) Silica and (2) Natural complex silicates.

  1. Silica (Silicon Dioxide, SiO2): This form occurs in substances like quartz, flint, sand, sandstone and agate.
  2. Natural Complex Silicates: These are combinations of SiO2 and metallic oxides such as [CaO, MgO, K2O, Al2O3, etc.]
    This form occurs in substances like,
    1. Feldspar: K2O. Al2O3. 6SiO2,
    2. Kaolin or China Clay: Al2O3. 2SiO2. 2H2O
    3. Mica: K2O. 3Al2O3. 6SiO2
    4. Asbestos: CaO. 3MgO. 4SiO2
    5. Talc: 3MgO. 4SiO2. H2O

 

 

4

Preparation of Silicon

 

Silicon is obtained in the amorphous and crystalline forms in three ways:

  1. From Silica: In absence of oxygen
  2. From Silicon Tetrachloride:
  3. From Potassium Silicofluoride:

 

 

5.

  Physical Properties of Silicon

 

Silicon exists in two allotropic forms:

  1. Amorphous and
  2. Crystalline
  1. The properties of amorphous form of silicon are:
    1. Brown powder having specific gravity of 2.35.
    2. Poor conductor of electricity.
    3. Chemically more reactive.


The properties of crystalline form of silicon are:

    1. Hard grey needle shaped crystals having specific gravity of 2.49.
    2. Conducts electricity feebly.
    3. Chemically less reactive.
  1. Common physical properties of both the forms:
    • Melting point of silicon is 1693K and boiling point is 2880K.
    • Silicon is insoluble in water and acids (except HF) but soluble in hot alkalis.
    • Silicon is a poor conductor of electricity at room temperature. However, conductance increases with temperature. Thus, silicon is a semiconductor.
    • Silicon is hard enough to produce a scratch on glass.
    • Silicon is a metalloid.

 

 

6.

Chemical Properties of Silicon

 

Following are the chemical properties of silicon:

(1) Reaction with water

(2) Reaction with alkali

  1. With aqueous alkali
  2. With fused alkali in the presence of air
    Si + 2NaOH + O2
    + H2O

(3) Reaction with sodium carbonate
Si + Na2CO3
+ C

(4) Reaction with metals

  1. With magnesium

    Si + 2Mg
  2. With copper

    Si + 2Cu
  3. With chromium

    3Si + 2Cr
  4. With sodium

    4Na + Si

(5) Reaction with halogens

  1. With fluorine

    Si + 2F2
  2. With chlorine

    Si + 2Cl2

 

 

7.

Uses of Silicon

 

  1. Silicon is used in the manufacture of semiconductors used in transistors.
  2. Silicon is also used as a deoxidiser in the preparation of castings of steel.
  3. Alloys like ferrosilicon, silicon bronze, silicon steel can be manufactured using silicon.
  4. Organosilicon compounds, halogenated silanes can be prepared using silicon.
  5. Crucibles can be prepared using silicon.

 

 

8.

Structure of Simple Silicates

 

Silicates are compounds of silicon with strong Si-O bonds. Most silicates are insoluble in water, while alkali metal silicates like sodium silicate Na2SiO3 are however soluble in water.

 

 

9.

Nature of the Si-O Bond and the Structure of SiO44 - Ion

 

  • The electronic configuration of Si in the ground state is 1s2, 2s2 2p6, 3s23p2.
  • The electronic configuration of Si in the excited state is 1s2 2s2 2p63s1 3px1 3py1 3pz1 .
  • The four sp3 hybrid orbitals of Si form four covalent bonds with p - orbitals of four oxygen atoms to form four Si-O bonds, i.e., sp3 - p bonds.
  • The four sp3 - p bonds are tetrahedrally arranged around the central Si atom.
  • Each oxygen atom accepts one electron to form silicate ion SiO44 -
  • The SiO44- unit is tetrahedral. The O - Si - O bond angle is 10928'.

 

 

10.

Strength of the Si - O Bond

 

  • The Si - O bond is very strong and stable.
  • The electronegativity of oxygen is 3.5 and that of silicon is 1.8; the difference in the electronegativity is 1.7. Thus, the Si - O bond, though covalent, has ionic character.
  • Only hydrofluoric acid (HF) can break the Si - O bond as the electronegativity of fluorine (4.0) is more than that of oxygen (3.5). Thus, the Si - F bond formed is more stable than Si - O bond.

 

 

11.

Types of Silicates

 

Silicates are metal salts of silicic acids. Example:

  1. Magnesium metasilicate (Enstatite) MgSiO3
  2. Zirconium orthosilicate (Zircon) ZrSiO4

 



 

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