Answers

2014-02-28T21:08:08+05:30
Its the matter of 2D and 3D

In diamond the carbon atoms are attached like 3D in a massive crystal frame however graphite has these solid bonds in 2D only.
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2014-02-28T22:07:44+05:30
The delocalised electrons in graphite are not attached to any carbon atom so are free to move and easily breakable further diamond is more tightly packed and has stable atomic structure 
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