Question

Question 11.29 What do you understand by

(a) inert pair effect

(b) allotropy and

(c) catenation?

Class XI The p-Block Elements Page 325

Answer 1

(a) inert pair effect : it is defined as the tendency of s - electrons to remain together or the reluctance of s - electrons to participate in a reaction , because the energy required to unpair the ns² electrons is not compensated by energy released in the forming two additional bonds .
ex :- Heavier members of group 13 , 14 and 15 exhibit an oxidation state two less than the number of valance shell electrons they posses e.g., Tl in +1 oxidation state is more stable than that of +3 oxidation state .

(b) Allotropy : The phenomena of existence of an element in two or more forms which differ in physical properties but have almost same chemical nature is known as Allotropy . This phenomena is due to the difference either in the number of atoms in the molecules {e.g., in O2 and O3} or arrangement of atoms in the molecules {in graphite , diamond and fullerenes}

(c) Catenation : The tendency of the Formation of long or closed chain by the combination of the same atoms is known as Catenation. Catenation property is maximum in Carbondale and decreases down the group .
e.g., C >> Si > Ge ≈ Sn >> pb

Answer 2

INERT PAIR EFFECT:

The inert pair effect is that the upper elements in a group has higher stability which decreses downward the group . For eg , in group 13 tellurium has +1 as more stable while aluminium has +3 as more stable oxidation state . Now the reason behind this is that in a group upper elements have smaller size , so during bond formation large amount of energy is released in comparison to lower elements which have a larger size and less effinity towards electron so less amount of energy is released during formation . The energy released during bond formation is used to take electrons out of atom , now in upper elements large energy is released so they excit there inner s orbital electrons while lower elements does not have sufficient energy to take s orbital electrons out of atom and hence they remain inside atom and act as inert pair . This is called inert pair effect.

ALLOTROPHY:

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond (the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations). The term allotropy is used for elements only, not for compounds. The more general term, used for any crystalline material, is polymorphism. Allotropy refers only to different forms of an element within the same phase (i.e.: solid, liquid or gas states); differences in these states alone would not constitute examples of allotropy.

For some elements, allotropes have different molecular formulae despite difference in phase; for example, two allotropes of oxygen (dioxygen, O2, and ozone, O3) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different phases; for example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

CATENATION:

(i) The tendency of formation of long open or closed atom chains by the combination of same atoms in themselves is known as catenation.

(ii) The catenation is maximum in carbon and decreases down the group.

(iii) This is due to high bond energy of catenation.

(iv) Only carbon atoms also form double or triple bonds involving pπ-pπ multiple bond within itself.

> C = C<; – C ≡ C –

(v) Carbon also possesses the tendency to form closed chain compounds with O, S and N atoms as well as forming pπ-pπ multiple bonds with other elements particularly nitrogen and oxygen e.g. C =O; C=N; C ≡ N; C = S are the functional groups present in numerous molecules due to this reason.

(vi) Carbon can form chain containing any number of carbon atoms Si and Ge cannot extend the chain beyond 6 atoms, while Sn and Pb do not form chains containing more than one or two atoms.

(vii) The reason for greater tendency of carbon for catenation than other elements in the group may further be explained by the fact that the C – C bond energy is approximately of the same magnitude as the energies of the bond between C and other elements. On the other hand, the Si – Si bond is weaker than the bond between silicon and other elements