ALLOTROPY:-
The existence of an element into two or more shapes or structure with different physical properties and similar chemical properties and similar chemical properties is called “Allotropic form” & this phenomenon is called “Allotropy”.
ALLOTROPY IN CARBON:-
Carbon exists into two forms:-
1- Crystalline Solid:- Such as diamond and graphite.
2- Amorphous Solid:- Such as coal, coke, carbon black etc.
1-DIAMOND: - Diamond is the crystalline allotropic form of carbon.
STRUCTURE OF DIAMOND:-
In diamond each carbon is covalently linked with other four carbon atoms to give basic tetrahedral unit. Tetrahedral units combine with another to produce cubic unit cell of diamond. The C-C atoms bond length is 1.54 A o or 154 pm &bond energy for each C-C bond is 347 Kj.mol/1.
PROPERTIES OF DIAMOND:-
i- It is the hardest known substances in the world due to carbon atoms forms grids-networks of & high bond energy.
ii- Diamond has very high refractive index i.e. 2.45 due to this reason diamond has great brilliance.
iii- Diamond is non-conductor of heat & electricity because it has no free electron of any carbon to conduct electricity.
iv- Diamond has density of 3.5 g/cm3.
v- Its melting point is 3500 o C .
USES OF DIAMOND:- It uses in jewellery and gems.
2-GRAPHITE: - Graphite is another crystalline allotropic form:-
STRUCTURE OF GRAPHITE:-
In graphite each carbon atoms id sp2 – hybridized and linked trigonally with other carbon atom. Each carbon atom in graphite uses 3 out of 4 electrons to form 3 covalent bond, and 1 electron remains free. In graphite carbon atom form plane sheets consisting of carbon atoms is deal localized and it can not help in making double bond.
The bond length of each C-C bond is 1.42 Ao which is intermediate value of single and double covalent bond. The plane sheets are held together with one another at a distance of 3.35 Ao by weak, Vender Wall forces of attraction. Due to this each layer is a slide over another.
PROPERTIES OF GRAPHITE:-
i- It is the dark grey crystalline solid.
ii- It possesses dull metallic lusture.
iii- It ha soft and greasy touch.
iv- It is lubricant.
v- Its density is 2.2 g/cm3 .
vi- It is a good conductor of electricity & used in preparation of electrodes.