Arsenic crystal structure, molecular model. Arsenic is a poisonous metalloid element. The unit cell (repeating structure) of the crystal lattice is shown in the yellow box, with atoms of arsenic shown as silver spheres. Arsenic is chemically similar to phosphorus, an essential component of body chemicals such as DNA and ATP (adenosine triphosphate, the body's energy storage molecule). Its ability to take the place of phosphorus in chemical reactions involving ATP is the source of its toxicity.