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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Third Row Elements Page 3

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6.

Properties of Metallic Solids

 

  • Malleability and Ductility
    The tendency of a metal to undergo flattening when hammered is called malleability.
    The tendency of a metal by which a wire can be drawn from it is called ductility.
    In a metal the metallic bond is non-directional. When force is applied to draw wires or hammer the metal cations and electrons adjust themselves and the metal bond remains intact. Hence, metals are malleable and ductile.
  • Thermal and Electrical Conductivity
    In a metal crystal, there are delocalised mobile electrons that can move freely. When an electric field is applied electrons conduct electricity. Thus, metals are good conductors of electricity. Similarly, when a metal is heated, these electrons absorb energy. When electrons collide with each other, this energy is then carried from one end of the metal to the other end. Thus, metals possess thermal conductivity.
  • Lustre
    The delocalised mobile electrons can absorb light of different wavelengths and re-emit the absorbed light. Hence, visible light is almost totally and effectively reflected. Metals therefore possess typical lustre and appear to be shining.

 

 

7.

Network Solids

 

  • Substances in which the atoms in the entire crystal lattice are held together by an extensive network of strong covalent bonds are called network solids.
  • Silicon is a network solid. The electronic configuration of silicon is 1s22s22p63s23p2. One of the 3s2 electrons is promoted to the vacant 3pz orbital to give the outermost configuration 3s13p3.
  • These four singly occupied orbitals mix to form the four equivalent sp3-hybridised orbitals in silicon atom. Each silicon atom is linked to four other silicon atoms by covalent bonds formed by the overlapping of two sp3 orbitals of two silicon atoms. This type of bonding leads to a giant network solid containing indefinite number of silicon atoms.
  • Silicon has a metallic appearance but, does not have free electrons and hence cannot conduct electricity at low temperature. As the temperature rises the electrical conductance increases. Thus, silicon is a semi-conductor.

 

 

8.

Molecular Solids

 

  • In a molecular solid, the molecular entities composed of atoms of the element are held together by vander Waal's forces.
  • Phosphorus, sulphur are molecular solids and chlorine, argon are gases at normal temperature and molecular solids at low temperature.
  • The electronic configuration of phosphorous is 1s22s22p63s23p3. The 3p orbitals being half filled, each phosphorus atom forms three covalent bonds with three other P atoms to form a P4 molecule. The P4 molecule has a pyramidal structure.
  • The electronic configuration of sulphur is 1s22s22p63s23p4. One sulphur atom can form two covalent bonds with the two half filled 3p orbitals. A sulphur molecule consists of eight atoms linked together in the form of a puckered ring structure.
  • The electronic configuration of chlorine is 1s23s22p63s23p5. There is only one unpaired electron in its 3p orbital. Thus, one chlorine atom can form a covalent bond with another chlorine atom to form a diatomic chlorine molecule (Cl2). Argon has completely filled outershell, hence is a monoatomic molecule.The vander Waal's forces of attraction are weak in the case of chlorine and argon. Thus, they are gases at normal temperature.

 



 

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