Carbongraphite form a less well packed giant hexagonal planar structure joined by Vanderwaals forces Its density 225gcm3 is therefore less than that of Carbondiamond353gcm3 and silicon233gcm3Both diamond and silicon have giant tetrahedral structure that is better packed Carbondiamond has smaller atomic radiussize than silicon Its density is thus higher because of better packing and subsequently higher density Carbondiamond is the hardest known natural substance by having the highest density