Arsenic has 42 neutrons. This gives it an atomic weight of around 75, which is determined by adding the 42 neutrons to arsenic's 33 protons. Arsenic is a poisonous gray solid.
Arsenic has been known since antiquity. Its name is derived from the Latin word arsenicum and the Greek word arsenikos. The German alchemist Albertus Magnus is thought to have been the first to isolate arsenic in 1250. Previously, arsenic compounds were mined by the ancient Greek, Egyptian and Chinese societies. Pure arsenic and its compounds are poisonous to most animals, including humans. This makes arsenic useful to the development of rat poison and several types of insecticides.