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P-Block Elements

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p Block Elements PDF Notes, Important Questions and Formulas

 The p-Block Elements

  • The elements in which the last electron enters the p-subshell of their outermost energy level are called p-block elements. The elements belonging to groups 13 to 18 constitute p-block elements.
  • General electric configuration for elements belonging to p-block is ns2np1-6
  • General characteristics of p-block elements:
    • Variation in oxidation states:

      Group

      13

      14

      15

      16

      17

      18

      General electronic configuration

      ns2np1

      ns2np2

      ns2np3

      ns2np4

      ns2np5

      ns2np6

      First member of group

      B

      C

      N

      O

      F

      He

      Group oxidation state

      +3

      +4

      +5

      +6

      +7

      +8

      Other oxidation states

      +1

      +2,-4

      -3,+3

      -2,+2,+4

      +5,+3,-1,+1

      +2,+4,+6

    • Metallic and non-metallic character: Elements which show characteristics of both metals and non-metals and are called metalloids. Common metalloids in p-block elements are Si, Ge, As, Sb, Te, Po and At.
Some important compounds of carbon and silicon:
  1. Carbon Monoxide (CO)
    1. Preparation of carbon monoxide (CO):
      begin mathsize 12px style table attributes columnalign left end attributes row cell 2 straight C left parenthesis straight s right parenthesis plus straight O subscript 2 left parenthesis straight g right parenthesis rightwards arrow with straight capital delta on top 2 CO left parenthesis straight g right parenthesis end cell row cell HCOOH rightwards arrow from conc. straight H subscript 2 SO subscript 4 to 373 straight K of straight H subscript 2 straight O plus CO end cell row cell straight C left parenthesis straight s right parenthesis plus straight H subscript 2 straight O left parenthesis straight g right parenthesis rightwards arrow with 473 minus 1273 straight K on top CO left parenthesis straight g right parenthesis plus straight H subscript 2 left parenthesis straight g right parenthesis end cell row cell text                                             Water gas end text end cell row cell text 2C(s)+O end text subscript 2 left parenthesis straight g right parenthesis plus 4 straight N subscript 2 left parenthesis straight g right parenthesis rightwards arrow with 1273 straight K on top 2 CO left parenthesis straight g right parenthesis plus 4 straight N subscript 2 left parenthesis straight g right parenthesis end cell row cell text                                                        Producer gas end text end cell end table end style   
    2. Properties of carbon monoxide (CO):

      1. It is a colorless, odourless and almost water insoluble gas.
      2. It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminum and a few transition metals.
        begin mathsize 12px style table attributes columnalign left end attributes row cell Fe subscript 2 straight O subscript 3 left parenthesis straight s right parenthesis plus 3 CO left parenthesis straight g right parenthesis rightwards arrow with straight capital delta on top 2 Fe left parenthesis straight s right parenthesis plus 3 CO subscript 2 left parenthesis straight g right parenthesis end cell row cell ZnO left parenthesis straight s right parenthesis plus CO left parenthesis straight g right parenthesis rightwards arrow with straight capital delta on top Zn left parenthesis straight s right parenthesis plus CO subscript 2 left parenthesis straight g right parenthesis end cell end table end style 
      3. CO is highly poisonous because it has the ability to form a complex with haemoglobin, which is about 300 times more stable than the ability to form a complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen around the body and ultimately resulting in death.
    3. Structure  of carbon monoxide (CO): In the CO molecule, there are one 𝜎 and two π bonds between carbon and oxygen: C=O:.  Because of the presence of a lone pair on carbon, CO molecule acts as a donor and reacts with certain metals when heated to from metal carbonyls.
      Carbon monoxide is regarded as the resonance hybrid of the following Structure:

  2. Carbon dioxide (CO2)
    (a) Preparation of carbon dioxide (CO2):
    begin mathsize 12px style table attributes columnalign left end attributes row cell straight C left parenthesis straight s right parenthesis plus straight O subscript 2 left parenthesis straight g right parenthesis rightwards arrow with straight capital delta on top CO subscript 2 left parenthesis straight g right parenthesis end cell row cell CH subscript 4 left parenthesis straight g right parenthesis plus 2 straight O subscript 2 left parenthesis straight g right parenthesis rightwards arrow with straight capital delta on top CO subscript 2 left parenthesis straight g right parenthesis plus 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis end cell end table end style
     CaCO3(s) + 2HCl (aq) → CaCl2 (aq) +CO2 (g) + H2O (l)
    (b) Properties of carbon dioxide (CO2):
    1. It is colorless and odourless gas
    2. With water, it forms carbonic acid H2CO3, which is a weak dibasic acid, and dissociate in two steps:
      H2CO3 (aq) + H2O (l) ⇌ HC(aq)+H3O+(aq)
      HCO3-(aq) + H2O (l) ⇌ CO(aq)+H3O+(aq)
      H2CO3/HCbegin mathsize 12px style blank subscript 3 superscript minus end style buffer system helps to maintain pH of blood between 7.26 and 7.42
    3. Being heavy and non-supporter of combustion carbon dioxide is used as a fire extinguisher.

    (c)   Structure of carbon dioxide:

    In a CO2 molecule, the carbon atom undergoes sp hybridization. Two sp hybridized orbitals of carbon atom overlap with two p-orbitals of oxygen atoms to make two sigma bonds, while other two electron of carbon atom are involved in pπ-pπ bonding with oxygen atom. This results in its linear shape having both C-O bonds of equal length (115 pm) with no dipole moment. The resonance structures are shown below.

  3. Silicon dioxide(SO2)
    Silicon dioxide is a covalent, three-dimensional network solid. In this structure, each silicon atom is tetrahedrally surrounded by four oxygen atoms. Each oxygen atom in turn is covalently bonded to other silicon atoms. Each corner is shared with another tetrahedron. The entire crystal may be considered as a giant molecule in which eight-membered rings are formed with alternate silicon and oxygen atoms. Thus, there are no discrete SiO2 units and it is a network solid. 

  4. Silicones
    Silicones are organosilicon polymeric compounds containing Si-O-Si linkages. These have the general formula (R2SiO-)n as a repeating unit. Starting materials for the manufacturing of silicones are alkyl- or aryl-substituted silicon chlorides, RnSiCl(4-n), where R is alkyl or aryl group. When methyl chloride reacts with silicon in the presence of copper as a catalyst at a temperature 573K various types of methyl substituted chlorosilane of formula MeSiCl3, Me2SiCl2 with small amount of Me4Si are formed. Hydrolysis of dimethyldichlorosilane, (CH3)2SiCl2 followed by condensation polymerisation yields straight chain polymers.

  5. Silicate:
    Basic building unit of all silicates is the tetrahedral SiO44-ion. In silicates, either the discrete unit is present or a number of such units are joined together via comes by sharing 1, 2, 3 or 4 oxygen atoms per silicate units. If all the four corner are shared with other tetrahedral units, then a three-dimensional network is formed.

  6. Zeolites:
    Aluminosilicates are formed when few Si atoms are replaced by Al atoms in a three- dimensional network of silicon dioxide. Zeolites are widely used as a catalyst in petrochemical industries for cracking of hydrocarbons and isomerization, e.g. ZSM-5 (a type of zeolite) used to convert alcohols directly into gasoline.
    Group 13 elements: The Boron family 

  • Electronic configurations: Elements of group 13 have the general outer electronic configuration of ns2np1

Elements

Symbol

Atomic No.

Electronic Configuration

Boron

B

5

[He]2s22p1

Aluminum

Al

13

[Ne]3s23p1

Gallium

Ga

31

[Ar]3d104s24p1

Indium

In

49

[Kr]4d105s24p1

Thallium

Tl

81

[Xe]4f145d106s26p1

 

  • Trends in properties of group 13 elements 

No.

Property

Variations along the group

1

Atomic radii

Atomic radii increases down the group but not regularly.

2

Ionization enthalpy

Ionization enthalpy decreases down the group but not regularly.

3

Electronegative

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

 

  • Some important compounds of boron 

No.

Name of compound

Formula of compound

1

Borax

Na2B4O7.H2O

2

Orthoboric acid

H3BO3

3

Diborane

B2H6

 Structure of diborane (B2H6) the electron diffraction studies have shown bridged structure for diborane as shown below:

Four hydrogen atoms (2 on the left and 2 on the right) are known as terminal hydrogens. These are different than the other two hydrogen atoms, which are known as bridged hydrogens. The two boron atoms and four terminal H-atoms lie in the same plane while the two bridging H-atoms are in a plane perpendicular to the rest of the molecule. There are two types of bonds n the molecule:

The four terminal hydrogen are bonded by normal covalent bonds formed by the sharing of one electron each from B and H atoms.

The bridged hydrogens are bonded by three centre electron pair bond, which involve one electron pair only, i.e. two electrons, but binds three atoms, i.e. two B atoms and one H atom. Boron atom undergoes Sp3 hybridisation involving 2s and all the three 2p orbitals including one empty orbital. The four sp3 hybrid orbitals adopt tetrahedral arrangement. Two hybrid orbitals of each B atom overlap with 1s orbital of two H atoms. Of the hybrid orbitals left on each B atom one contains an unpaired electron, while the other is vacant. The hybrid orbital containg an unpaired electron of one B atom and the vacant hybrid orbital of the second B atom overlaps simultaneously with 1s orbitals of H atom to form B-H-B bridge bond. The B-H-B bond is called a three centre electron pair bond.

Uses of Borax:

  1. In making glass, enamel and gaze or pottery.
  2. As antiseptics in medical soaps preparation.
    Al2O3 preparation

    begin mathsize 12px style table attributes columnalign left end attributes row cell left parenthesis straight i right parenthesis space 2 Al stack open parentheses OH close parentheses subscript 3 rightwards arrow with 300 degree straight C on top Al subscript 2 straight O subscript 3 plus 3 straight H subscript 2 straight O end cell row cell left parenthesis ii right parenthesis space Al subscript 2 space open parentheses SO subscript 4 close parentheses subscript 3 rightwards arrow with straight capital delta on top Al subscript 2 straight O subscript 3 plus 3 SO subscript 3 end cell row cell left parenthesis iii right parenthesis space open parentheses NH subscript 4 close parentheses subscript 2 SO subscript 4. Al subscript 2. open parentheses SO subscript 4 close parentheses subscript 3 24 straight H subscript 2 straight O rightwards arrow with straight capital delta on top Al subscript 2 straight O subscript 3 plus 2 NH subscript 3 plus 4 SO subscript 3 plus 25 straight H subscript 2 straight O end cell end table
end style
Uses:
  1. In making refractory brick
  2. As abrasive
  3. To make high alumina cement

Group 14 element carbon family

  1. Electronic configuration: Elements of group 14 have the general outer electronic configuration of ns2np2

Element

Symbol

Atomic Number

Electronic Configuration

Carbon

C

6

[He]2s22p2

Silicon

Si

14

[Ne]3s23p2

Germanium

Ge

32

[Ar]3d104s24p2

Tin

Sn

50

[Kr]4d105s25p2

Lead

Pb

82

[Xe]4f145d106s26p2

 

  1. Trends in properties of group 14 elements:

No

Property

Variation along the group

1

Atomic radii

An atomic radius increases down the group. There is a considerable increase in covalent radius from C to Si, thereafter from Si to Pb a small increases in radius is observed

2

Ionization enthalpy

Ionizarion enthalpy decreases down the group but not regularly. Small decrease in ∆1H from Si to Ge to Sn and slight increases in ∆1H from Sn to Pb is the consequence of poor shielding effect of intervening d- and f- orbitals and increase in size of the atom

3

Electronegativity

Due to their small size, the element of this group are slightly more electronegative than group 13 elements. The electronegative values for elements from Si to Pb are almost the same.

iii. Allotropes of carbon: Allotrope forms or allotropes are the different forms of the same elements having different physical properties but similar chemical properties.

  1. Crystalline form: Diamond and graphite are two crystalline forms of carbon having a well-defined structure
  2. Amorphous form: Coal, wood charcoal, animal charcoal, animal charcoal, lamp black, coke etc. are many amorphous forms of carbon.
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