Arsenic

Arsenic is a chemical element with the symbol As and atomic number 33. It belongs to the group known as metalloids—substances that are chemically intermediate between metals and nonmetals. Arsenic has several allotropic forms, the most common being a brittle, crystalline solid that resembles metal.

In nature, arsenic is rarely found in pure form, but occurs in minerals such as arsenopyrite. It is also present in groundwater in some parts of the world, which is a significant public health concern. Arsenic compounds have been known since ancient times, and their use is now strictly regulated.

Chemical properties

Arsenic does not melt under normal atmospheric pressure, but instead sublimes directly from solid to gas. It forms compounds with oxygen, sulfur, and halogens. The most common compound is arsenic trioxide (As₂O₃), a white powder that is highly toxic. Arsenic exhibits multiple oxidation states, including −3, +3, and +5; the +3 state is generally the most toxic.

Health effects

Inorganic arsenic compounds are recognized as toxic and are associated with cancer. Prolonged exposure to water containing arsenic causes skin changes, nerve damage, and increased cancer risk, particularly of the skin, lungs, and bladder. Acute exposure to high doses causes gastrointestinal illness, vomiting, and diarrhea.

Uses

The principal industrial use today is in electronics: gallium arsenide (GaAs) is used in high-speed semiconductors, LEDs, and solar cells. Historically, arsenic was added to medicines, dyes, and treatments for syphilis and pest control. Most of these uses have been reduced or discontinued for health and environmental reasons.

History

Arsenic compounds have been known since antiquity, particularly colored sulfides such as orpiment and realgar, which were used as pigments. The isolation of pure arsenic is credited to medieval alchemists. Because of its extreme toxicity and lack of odor or taste when dissolved, arsenic features prominently in historical accounts of poisonings and murders, until chemical detection methods such as the Marsh test were developed.