 Elemental antimony adopts a layered structure ( space group <unk> No. 166 ) in which layers consist of fused <unk> six @-@ membered rings . The nearest and next @-@ nearest neighbors form an irregular <unk> complex , with the three atoms in the same double layer being slightly closer than the three atoms in the next . This relatively close packing leads to a high density of 6 @.@ 697 g / cm3 , but the weak bonding between the layers leads to the low hardness and <unk> of antimony . 
