English: Arsenic, which is known since ancient times, sometimes natively occurs in nature as a grey metal. In its compounds it is one of the most toxic elements. Arsenic trioxide, As2O3, was for many centuries the most popular poison for assassination. But arsenic also was and is still used as a pharmaceutical and was the main ingredient in the first chemotherapy. Today, it is mainly used in lead alloys and for special semiconductors. The handling of arsenic is always very dangerous and needs special safety precautions. A deadly dose is about one tenth of a gram.