The Arsenic Group is composed of semi-metallic elements and one alloy that have the same crystal structure as arsenic. The group is composed of only four minerals: antimony (Sb), bismuth (Bi), stibarsen (SbAs) and of course arsenic (As).
The overall structure is composed of sheets of connected atoms, each atom bonded to three other atoms within the sheet, but not strongly bonded to atoms above or below it. Each atom is either at the top of a pyramid with its three bonded atoms below it or it is at the bottom of a pit with the three bonded atoms above it. The trigonal symmetry is caused by the trigonal nature of the bonding and the sheet structure is the cause of the perfect basal cleavage seen in the members of this group.