Arsenic (As)

Arsenic is the chemical element with atomic number 33 and symbol $\mathrm{As}$. It is a metalloid. Its chemistry includes oxidation states $-3$, $+3$, and $+5$, and it occurs in both inorganic and organic compounds. Chemical form matters: elemental arsenic, arsine, and arsenic oxides differ in bonding and physical behavior.^[ref]

An arsenic compound contains arsenic chemically combined with other elements. Examples include covalent arsine, arsenic trioxide, and arsenic trisulfide.

The oxidation state and chemical partners strongly affect behaviour. Elemental composition alone does not specify volatility, solubility, or biological effects, so identifying an arsenic-containing material requires its chemical form as well as the amount of arsenic.

Arsine is $\mathrm{As}{\mathrm{H}}_3$, a molecular hydride of arsenic with a pyramidal arrangement of three hydrogen atoms around arsenic. It is the arsenic analogue of Ammonia, although their chemical properties differ substantially.

The small molecular structure gives arsine a very different physical form from solid arsenic oxides or sulfides. This illustrates why an element's compounds cannot be assigned the properties of the pure element.

Arsenic trisulfide is $\mathrm{A}{\mathrm{s}}_2{\mathrm{S}}_3$, an arsenic-sulfur compound. Its crystalline mineral form is orpiment; amorphous forms have different structural order.

The distinction between a crystal and an amorphous solid matters for optical behaviour and material processing. The same nominal elemental ratio can therefore occur in materials with different local structure and physical properties.

Arsenic trioxide is the arsenic(III) oxide conventionally written $\mathrm{A}{\mathrm{s}}_2{\mathrm{O}}_3$. The formula expresses its elemental ratio; molecular units in some forms are $\mathrm{A}{\mathrm{s}}_4{\mathrm{O}}_6$.

This distinction separates an empirical formula from the structure of the actual species. Reactions involving arsenic trioxide depend on pH, dissolved speciation, and oxidation state, rather than on the oxide formula alone.

The hydrated arsenic(III) compound $\mathrm{As}(\mathrm{OH}{)}_3$; distinguish it from the historical dry product called arsenious acid, arsenic trioxide.

Historical pharmaceutical preparation; its composition and clinical use must be identified by edition. ^[ref]

The +3 oxidation state of arsenic. The actual aqueous species can depend on pH, hydrolysis and ligands; a bare ionic formula is an analytical shorthand.