Britannica Sources used in this answer

How does elemental arsenic react with dry air?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A labeled diagram of the periodic table entry for arsenic, showing its atomic number 33, symbol As, atomic weight 74.921595, electron configuration, and classification as an other nonmetal. Additional icons indicate arsenic is solid at room temperature, has a rhombohedral crystal structure, and weakly acidic oxides. — arsenic Properties of arsenic.

Elemental arsenic is stable when exposed to dry air. In contrast, it tends to become coated with a black oxide when exposed to moist air.1