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Saturday, 11 February 2017

The p-block elements V

Anomalous properties of boron
1. Boron is a non-metal & bad conductor of electricity whereas aluminium is a metal & good conductor. B is hard but Al is a soft metal.
2. Boron exists in two forms-crystalline and amorphous. But Al does not exist in different forms
3. The melting and boiling point of boron are much higher than that of Al .
4. Boron forms only covalent compounds whereas Al forms even some ionic compounds.
5. The hydroxides and oxides of boron are acidic in nature whereas those of aluminium are amphoteric.
6. The trihalides of boron exist as monomers. On the other hand, aluminium halides exist as dimers .
7. The hydrides of boron are quite stable while those of aluminium are unstable

• Boron and silicon exhibit the typical properties of non-metals. These do not form cations. Both exist in amorphous as well as crystalline forms.

• Boron oxide (B2O3 and silica ((SiO2)) both are acidic and dissolve in alkali solutions to form borates and silicates respectively.
B2O3  + 6NaOH → 2Na2BO3 + 3H2O

SiO2 +  2NaOH → Na2SiO3   + H2O

• The chlorides of both B and Si get hydrolyzed by water to boric acid and silicic acid respectively.

BCl3  + 3H2O  → H3BO3 + 3HCl                     
SiCl4  + 3H2O  → H2SiO3 + 4HCl
  • The hydrides of Boron and Silicon are quite stable. Numerous volatile hydrides are also known which catch fire on exposure to air and are easily hydrolyzed.
  • Both elements are semiconductors.
Behavior in Aqueous Solutions
1. Al, Ga, In and Tl exhibit a well-defined aqueous chemistry in their tripositive states.
Species like M(OH)4]-, [M(H2O)2(OH)4]-, [M(OH2)6]3+ for M = Al, Ga, In, exist in aqueous solution.

2. Al, Ga. In and T1 ions exist as octahedral aqua ions, [M(OH2)6]3+ in aqueous solution and many salts like halides, sulphates, nitrates and perchlorates exist as hydrates.

3. Aluminiumsulphate forms double salts - called alum, having the general formula
     M2SO4. Al2(SO4)3.12H2O, where M = Na+ or K+.

USES OF BORON & ALUMINIUM :
  • Aluminium is used extensively in industry and everyday life. It forms many useful alloys with Cu. Mn, Mg, Si and Zn. Hence, aluminium and its alloys find use in packaging, utensil making, construction, aerospace and other transportation industries. It is used as a conductor for transmission of electricity. Aluminium is alsoused in the alumino-thermite process for production of chromium and manganese from their ores.
  • Group 14 Elements:-The Carbon Family
  • Group 14 includes carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb).
  • General electronic configuration of carbon family is ns2np2.
Covalent radius:- Covalent radius expected to increase from C to Si, From Si to Pb small increase is found.

Ionization Enthalpy:- The first ionization enthalpies of group 14 elements are higher than those of the corresponding group 13 elements.

Electronegativity:- Group 14 elements are smaller in size as compared to group 13 elements that’s why this group elements are slightly more electronegative than group 13

Chemical properties:-
  • Carbon and silicon mostly show +4 oxidation state. Germanium forms stable compounds in +4 state and only few compounds in +2 state.
  • Tin forms compounds in both oxidation states. Lead compounds in +2 state are stable and in +4 state are strong oxidizing agents.
Exception:- Pb4 and SnF4 are ionic in nature.
Except CCl4 other tetrachlorides are easily hydrolysed by water.
Since carbon does not have d-orbitals and hence cannot expand its coordination number beyond 4
                               CCl4 +H2O   --------> No Reaction
                                          SiCl4+4H2O  --------> Si(OH)+ 4HCl
                                                                       Silicic acid
Allotropes of Carbon:- The three types of allotropes are
1-Diamond
2-Graphite
3-Fullerence

Diamond:- In diamond each carbon atom undergas SP hybridisation. Each carbon is tetrahedrally linked to four other carbon atoms.

Graphite:- In graphite, carbon is SPhyberdized graphite has a two-dimensional sheet like structure consisting of a number of hexagonal rings fused together.

Graphite conducts electricity along the sheet. It is very soft and Slippery Fullerence Fullerence was discovered collectively by three scientists namely R.E Smalley,R.F Curl and H.W Kroto

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