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The p-Block Elements



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Chapter 11 : The p-Block Elements



11.1 Group 13 Elements: The Boron Family arrow_upward


  • Boron is a fairly rare element, mainly occurs as orthoboric acid, (H3 BO3 ), borax, Na2 B4 O7 10H2 O, and kernite, Na2 B4 O7 4H2 O.

  • 11.1.1 Electronic Configuration

  • The outer electronic configuration of these
  • elements is ns2 np1 .

  • 11.1.2 Atomic Radii

  • On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase.

  • 11.1.3 Ionization Enthalpy

  • The ionisation enthalpy values as expected
  •      from the general trends do not decrease  
  •      smoothly down the group.
  • The decrease from B to Al is associated with increase in size.

  • 11.1.4 Electronegativity

  • Down the group, electronegativity first decreases from B to Al and then increases.

  • 11.1.5 Physical Properties

  • Boron is non-metallic in nature. It is extremely hard and black coloured solid.

  • 11.1.6 Chemical Properties

  • Reactivity towards acids and alkalies: Boron does not react with acids and alkalies even at moderate temperature; but aluminum dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.
  •                   


    11.2 Important Trends and Anomalous Properties of Boron arrow_upward


  • The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as NH3 to complete octet around boron.
  •                F3 B  +  :NH3 ----> F3 B<----NH3

  • 11.3 Some Important Compounds of Boron arrow_upward


  • Some useful compounds of boron are borax, orthoboric acid and diborane.

  • 11.3.1 Borax

  • It is the most important compound of boron.
  • It is a white crystalline solid of formula Na2 B4 O7 ⋅10H2 O.
  • In fact it contains the tetra nuclear units and correct formula; therefore, is Na2 [B4 O5 (OH)4 ].8H2 O. Borax dissolves in water to give an alkaline solution.
  •                                                                                                                                                          


    11.3.2 Orthoboric acids

  • Orthoboric acid, H3 BO3 is a white crystalline solid, with soapy touch. It is sparingly soluble in water but highly soluble in hot water.
  •  It can be prepared by acidifying an aqueous solution of borax.
  • Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:

  • 11.3.2 Diborane

  • The simplest boron hydride know, is diborane.
  • It is prepared by treating boron trifluoride with LiAlH4 in diethyl ether.
  • The structure of diborane, B2 H6
  •         


    11.4 Uses of Boron and Aluminum and their Compounds arrow_upward


  • Boron being extremely hard refractory solid of high melting point, low density and very low electrical conductivity.
  • Boron fibres are used in making bullet-proof vest and light composite material for aircraft.
  • The main industrial application of manufacture of heat resistant glasses.

  • 11.5 Group 14 Elements: The Carbon Family arrow_upward



    Property

    Carbon

    C

    Silicon

    Si

    Germanium

    Ge

    Tin

    Sn

    Lead

    Pb

    Atomic Number

    6

    14

    32

    50

    82

    Electronic configuration

    [He]2s2

    2p6

    [Ne]3s2

    3p6

    [Ar]3d10 4s2

    4p6

    [Kr]4d10 5s2

    5p2

    [Xe]4f14 5d6s2

    6p2



    11.5.1 Electronic Configuration

  • The valence shell electronic configuration of
  • these elements is ns2 np2 .
  •                                                    

  • 11.5.2 Covalent Radius

  • There is a considerable increase in covalent
  • radius from C to Si, thereafter from Si to Pb a small increase in radius is observed.

  • 11.5.3 Ionization Enthalpy

  • The first ionization enthalpy of group 14 members is higher than the corresponding members of group 13.

  • 11.5.4 Electronegativity

  • Due to small size, the elements of this group are slightly more electronegative than group 13 elements.

  • 11.5.6 Physical Properties

  • All group 14 members are solids. carbon and silicon are non-metals, germanium is a metalloid, tin and lead are soft metals with low melting points.

  • 11.5.7 Chemical Properties

  • Reactivity towards water: Carbon, silicon and germanium are not affected by water. Tin decomposes steam to form dioxide and dihydrogen gas.
  •                                


    11.6 Important Trends and Anomalous Properties of Carbon arrow_upward


  • Carbon also has unique ability to form pπ– pπ multiple bonds with itself and with other atoms of small size and high electronegativity.
  • Few examples of multiple bonding are: C=C, C ≡ C, C = O, C = S, and C ≡ N. Heavier elements do not form pπ– pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.
  • Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings.

  • 11.6 Allotropes of Carbon arrow_upward


  • Carbon exhibits many allotropic forms; both
  • crystalline as well as amorphous.
  • Diamond
  • In diamond each carbon atom undergoes sp 3 hybridization and linked to four other carbon atoms by using hybridized orbitals in  tetrahedral fashion, The C–C bond length is 154 pm.
  •             

    Graphite
  • Graphite has layered structure and distance between two layer is 340pm.
  • Each layer is composed of planar hexagonal rings of carbon atoms.
  • Graphite conducts electricity along the sheet.

  • 11.8 Some Important Compounds of Carbon and Silicon arrow_upward


    Oxides of Carbon
  • Two important oxides of carbon are carbon monoxide, CO and carbon dioxide, CO2 .

  • 11.8.1 Carbon Monoxide

  • Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide.
  • When air is used instead of steam, a mixture of CO and N2 is produced, which is called producer gas.
  •                                                                                                                                         


    11.8.2 Carbon Dioxide

  • It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.
  •                 

    • In photosynthesis process green plant convert atmospheric CO2 into carbohydrates such as glucose.


    11.8.3 Silicon Dioxide, SiO2

  • Silicon dioxide, commonly known as silica, occurs in several crystallographic forms.
  • Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms
  • Three-dimensional structure of SiO2 .

  • 11.8.4 Silicones

  • They are a group of organosilicon polymers.
  • The manufacture of silicones is alkyl or aryl substituted silicon chlorides.

  • 11.8.5 Silicates

  • A large number of silicates minerals exist in nature. Some of the examples are feldspar, zeolites, mica and asbestos.
  • The basic structural unit of silicates is SiO4 4– .

  • 11.8.6 Zeolites

  • Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerisation, e.g., ZSM-5.


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