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Solutions/Answers to NCERT/CBSE CHEMISTRY Class 11(Class XI)textbook
CHAPTER ELEVEN
The p-Block Elements
11.25 What are allotropes? Sketch the structure of two allotropes of carbon
namely Diamond and graphite. What is the impact of structure on physical
properties of two allotropes?
When an element exists in two or more forms having different physical
property, but same chemical properties, the phenomenon is called as
allotropy and the various forms are called as allotropes. Example Diamond
and graphite.
The structure and properties of diamond is as given below.
(1) Each carbon atom is covalently bonded with four other carbon atoms.
(2) These four atoms are again strongly bonded with other four carbon atoms.
(3) All the bonds are sp3 hybridized. The structure formed is therefore very rigid.
This makes diamond one of the hardest substances.
(4) Diamond shows very high melting point (about 3500°C). High amount of heat
has to be introduced to break the bonds; hence
Diamond shows very high melting point.
(5) Diamond is compact and has high density of about 3.5 gm/cm3.
(6) The electrons are completely shared by neighboring carbon atoms and are
not “free” Therefore diamond is not a good conductor
of electricity.
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(7) Diamond is a good conductor of heat because the electrons in the
bonds are shared and hence the heat can be dissipated easily
The structure and properties of Graphite is as given below
(1) The hexagonal form of graphite has carbon atoms arranged in a hexagon; the
hexagons form a plane.
(2) Each carbon atom in the hexagon is attached to only three other carbon
atoms. The fourth electron of each carbon atom hangs loosely and is not
bound.
(3) Graphite shows sp2 hybridization.
(4) Within each layer, the carbon atoms are very strongly bonded. The links
between adjacent layers is weak .The sheet like structure with weak interlayer forces makes graphite a very soft substance.
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(5) The melting point of graphite is relatively high (37000 C). This is because of
the strong overlap of the sigma bonds within the carbon atoms in the planes.
Sigma bonds are strong and they need a large amount of energy to break.
Hence, graphite has a high melting point.
(6) Graphite is a good conductor of heat and electricity. This is because of the
loosely bound fourth electron of each carbon atom. Which is free to move
about.
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