Arsenic is an element (atomic number 33) classed as a semi-metal or metalloid. This means it has some properties of metals, and some properties of non-metals. Arsenic occurs in two distinct solid forms. One is a brittle, gray metal, while the other is a yellow, non-metallic form, rarely seen outside the laboratory. Arsenic and its compounds often have a garlic-like odor when crushed or when scratched with a hard object.

Elemental arsenic has very few uses. Nearly all the applications are as salts or oxides of arsenic. Arsenic compounds can be very toxic, and their uses are strictly controlled by health and environmental regulations.