Arsenic (pronounced /ˈɑrsnɪk/; also /ɑrˈsɛnɪk/ when attributive) is the chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250. Its atomic mass is 74.92. Arsenic is a notoriously poisonous metalloid with many allotropic forms, including a yellow (molecular non-metallic) and several black and grey forms (metalloids). Three metalloidal forms of arsenic, each with a different crystal structure, are found free in nature (the minerals arsenic sensu stricto and the much rarer arsenolamprite and pararsenolamprite). However, it is more commonly found as arsenide and in arsenate compounds, several hundred of which are known. Arsenic and its compounds are used as pesticides, herbicides, insecticides and in various alloys.

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