Arsenic

A metalloid infamous as a poison, present in groundwater around the world, and also a modern leukemia drug.

At a glance

Symbol: As
Atomic number: 33
Average atomic mass: 74.92
Group: 15
Period: 4
Family: metalloid
Electron configuration: [Ar] 3d¹⁰ 4s² 4p³
Valence electrons: 5
Electronegativity: 2.18
Common ions: −3 in arsenides; +3 and +5 with oxygen, in arsenites and arsenates

On the table

Period 4, group 15. The other metalloids are tinted, and every cell opens its own article.

Electrons and bonding

Five valence electrons, like nitrogen and phosphorus. Arsenic mostly shares electrons in covalent bonds, taking the +3 or +5 state with oxygen.

Properties

State at 25 °C: solid
What it looks like: brittle, metallic gray solid
Sublimes: at 615 °C, going straight from solid to gas
Density: 5.78 g/cm³

Where it is found

Occurs in many minerals, especially arsenopyrite (FeAsS), and as the colorful sulfides realgar and orpiment. Most is recovered while smelting copper and gold ores. Arsenic dissolved from rock contaminates well water in Bangladesh, India and other regions.

Discovery and name

Arsenic compounds were known in the ancient world as pigments and poisons, and Albertus Magnus is credited with isolating the element around 1250. The name comes through the Greek arsenikon from the Persian zarnikh, yellow orpiment.

What it is used for

Gallium arsenide semiconductors, and hardening the lead in car batteries and ammunition. It was once widely used in wood preservatives, pesticides and green pigments such as Paris green. Arsenic trioxide is an effective treatment for one type of leukemia, acute promyelocytic leukemia.

In compounds

As₂O₃arsenic trioxide, a poison and a medicineGaAsgallium arsenideAs₄S₄realgar, redAs₂S₃orpiment, yellowAsH₃arsine, a toxic gas

Handle with care

Arsenic and its compounds are toxic and cause cancer. It was so common a murder weapon that it was nicknamed inheritance powder, until the Marsh test of 1836 made it easy to detect.