Allotropy is the property by virtue of which an element exist in more than one form and each form has different physical properties but identical chemical properties. These different forms are called allotropes. The two common allotropic forms of carbon are diamond and graphite.

Carbon exists in two allotropic forms:(1) CRYSTALLINE FORM(2) AMORPHOUS FORM

CRYSTALLINE FORMS

There are three crystalline forms of carbon:(1) Diamond(2) Graphite(3) Bucky ball

AMORPHOUS FORMS

There are various amorphous forms of carbon such ascoalcokecharcoallampblackgas carbon etc.

DIAMOND

In diamond, each C-atom is covalently bonded to four other C-atom to give a tetrahedral unit. In diamond each C-atom is sp3-hybridized.Therefore each C-atom forms four sigma bonds withneighbouringC-atoms.In diamond C-C-C bond angle is 109.5O.These basic tetrahedral units unite with one another and produce a cubic unit cell.C-C bond length in diamond is 1.54AO.C-C bond energy is 347 kj/mole.In diamond crystal, basic units joined to forms octahedral shape of diamond crystal.

PROPERTIES OF DIAMOND

In diamond each C-atom utilizes its four unpaired electrons in bond formation. These bonding electrons are localized. Due to this reason diamond is a bad conductor of electricity.Diamond is the hardest substance ever known.Pure diamond is cloudless.Its melting point is 3500OC.

Pure diamond is transparent to x-rays.It has high refractive index i.e. 2.45.Due to impurities it may be colored.Its density is 3.5 gm/cm3.

GRAPHITE

In graphite each C-atom is covalently bonded to three C-atom to give trigonal geometry. Bond angle in graphite is 120O. Each C-atom in graphite is sp2-hybridized. Three out of four valance electrons of each C-atom are used in bond formation while the forth electrons free to move in the structure of graphite. Basic trigonal units unite together to give basic hexagonal ring. In hexagonal ring C-C bond length is 1.42AO. In graphite these rings forms flat layers. These layers are arranged is parallel ,one above the other. These layers 3.35AOapart and are held together by weak van der waals forces only.These layers can slide over one another.Thus it is very soft. Forth electron of each c-atom forms delocalized

p-system.

PROPERIES OF GRAPHITE

It is dark gray, have dull metallic luster.It is soft and greasy.It is used as lubricant.Its density is 2.2 gm/cm3.It is used in the preparation of electrodes as it canduct electricity.It is used as "pencil lead".It is used as moderator in nuclear reactors.