11th Class Chemistry The p-Block Elements-I

  • question_answer 37) What do you understand by: (a) inert pair effect (b) allotropy (c) catenation?


    (a) Inert pair effect: p-block elements have general electronic configuration \[n{{s}^{2}}\cdot n{{p}^{1-6}}\]. The tendency of s-electrons of valence shell to participate in bond formation decreases as we move down the group. This reluctance of s-electrons is termed inert pair effect. It is due to poor shielding of \[n{{s}^{2}}\]electrons by intervening d-and f-electrons. Inert pair effect is the property of only p-block elements. (b) Allotropy :(The property due to which an element exists in two or more forms which differ in their physical and some of the chemical properties is known as allotropy and the various forms are called allotropes or allotropic modifications. This phenomenon is due to the difference either in the number of atoms in the molecules or in the arrangement of atoms in the molecules.) Carbon's allotropic forms are: Diamond, graphite, coal, charcoal and lamp black. Diamond is the purest form of carbon and is very hard. It is a nonconductor of electricity. It is extremely chemically inactive. Graphite, on the other hand, is less dense than diamond, soft in nature and is a good conductor of electricity. Coal, charcoal and lamp black are amorphous forms. Coal is the crude form of carbon. Fullerenes, a new family of carbon allotropes, has been discovered in1985. These allotropes consist of clusters of carbon atoms such as\[{{C}_{50}},{{C}_{60}},{{C}_{70}},{{C}_{84}}\], etc. Silicon also exists in two forms: crystalline (grey) and amorphous (brown) forms. Amorphous silicon is chemically more active. Germanium has two allotropic forms while tin exists in three forms: grey tin, white tin and rhombic tin. (c) Catenation: The linking of identical atoms with each other to form long chains is called catenation. All the elements of this group have the property of catenation. However, this property decreases from carbon to lead. Thus, carbon has the maximum property of catenation, silicon has much lesser tendency, germanium has still lesser tendency whereas tin and lead hardly show this property. The decrease of this property is associated with M-M bond energy which decreases from carbon to lead.   \[CC\] 83 kcal/mol or 348 kJ/mol \[SiSi\] 54 kcal/mol or 297 kJ/mol \[GeGe~~\] 40 kcal/mol or 260 kJ/mol \[SnSn~~~~\] 37 kcal/mol or 240 kJ/mol   The reason for greater tendency of carbon for catenation than other elements may further be explained by the fact that C-C bond energy is approximately of the same value as the energies of bonds between carbon and other elements. On the other hand Si-Si bond is weaker than the bonds between silicon and other elements.   CC 83 kcal/mol SiSi 54 kcal/mol CO 86 kcal/mol SiO 88 kcal/mol CCl 81 kcal/mol SiC 186 kcal/mol   Thus, carbon forms a number of compounds in which a large number of carbon atoms are linked together in the form of straight chains, branched chains or closed rings. The property of catenation is responsible for a very large number of compounds of carbon.  

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