Category: Metalloid
Atomic Number: 33
Period: 4 | Group: 15
Block: p-block
The classic poison of history — tasteless, undetectable for centuries, and still a groundwater crisis for millions.
| Property | Value |
|---|---|
| Phase at STP | Solid |
| Density | N/A |
| Melting Point | 1090.00 K (816.85 °C) |
| Boiling Point | 889.00 K (615.85 °C) |
| Heat Capacity | N/A |
| Property | Value |
|---|---|
| Atomic Mass | 74.9216 u |
| Electronegativity (Pauling) | N/A |
| Oxidation States | -3, +2, +3, +5 |
| Electron Configuration |
| Discovered By | Unknown |
|---|---|
| Discovery Year | Ancient |
| Named After | Greek arsenikon, yellow orpiment |
Known since antiquity
The arsenic sulfides orpiment and realgar were used as pigments and medicines in ancient Egypt, Greece, China and India. The name comes from the Greek arsenikon, the word for yellow orpiment, which probably derives from a Persian word for gold-colored.
Albertus Magnus (around 1250)
The German scholar Albertus Magnus is credited with first isolating the element, around 1250, by heating orpiment with soap. In 1649 Johann Schröder published two methods for preparing it.
The poisoner's poison
Arsenic trioxide is tasteless and looks like sugar or flour, which made it a notorious poison from the Middle Ages to the 19th century. In 1836 the British chemist James Marsh developed a sensitive test for arsenic in body tissues, and reliable detection helped end its use for murder.
Recognized as an element
Antoine Lavoisier included arsenic in his 1789 list of chemical elements.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| As-75 | 42 | 100% | Stable | - |
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Semiconductors
Very pure arsenic is combined with gallium to make gallium arsenide (GaAs), a semiconductor used in LEDs, laser diodes, radio-frequency chips in mobile phones and high-efficiency solar cells on satellites. Arsenic is also used in small amounts as a dopant in silicon chips.
Wood preservation and alloys
Wood preservative: chromated copper arsenate (CCA) protected outdoor timber against rot and insects. Since 2004 it has no longer been used for residential timber in the United States and Canada, but it is still used for utility poles and some industrial wood.
Lead alloys: small additions of arsenic harden lead for battery grids and ammunition.
Medicine
Arsenic trioxide is a standard treatment for acute promyelocytic leukemia, approved in the United States in 2000. Earlier, Salvarsan, an arsenic compound introduced by Paul Ehrlich in 1910, was the first effective treatment for syphilis.
Former uses
Arsenic was once widely used in pesticides and herbicides, in pigments such as Scheele's green and Paris green, and in glassmaking. Most of these uses have ended because of its toxicity.
| Earth's Crust | N/A |
|---|---|
| Universe | N/A |