Answer:
(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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