Arsenic (As)

Arsenic was the poison of choice for centuries because arsenic trioxide is tasteless, roughly the colour of sugar, and produced symptoms indistinguishable from cholera — until the Marsh test in 1836 made it detectable in a corpse, which effectively ended the era. It also has a genuine modern use in medicine: arsenic trioxide, combined with a vitamin A derivative, cures the great majority of patients with acute promyelocytic leukaemia. Tens of millions of people in Bangladesh and West Bengal drink groundwater with naturally elevated arsenic, an accidental mass poisoning caused by tube wells drilled to avoid surface-water disease.

Properties

Atomic number

33

Symbol

As

Standard atomic weight

74.922 u

Category

Metalloid

Group

15

Period

4

Block

p-block

Electron configuration

[Ar] 4s² 3d¹⁰ 4p³

Electrons per shell

2, 8, 18, 5

Melting point

1,090 K — 816.9 °C — 1502.3 °F

Boiling point

887 K — 613.9 °C — 1136.9 °F

Density

5.776 g/cm³

Electronegativity

2.18 (Pauling)

Oxidation states

+5, +3, -3

Discovered

Known since antiquity

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³

Noble gas shorthand

[Ar] 4s² 3d¹⁰ 4p³

Shells

K: 2 · L: 8 · M: 18 · N: 5

What it is used for

• Gallium arsenide chips in phones and satellites
• Doping silicon to make n-type semiconductors
• Lead-acid battery grids, where a little arsenic hardens the lead
• Arsenic trioxide as a treatment for acute promyelocytic leukaemia
• Wood preservative, now largely phased out of residential use

Hazards

• Acutely toxic; the lethal dose of arsenic trioxide is on the order of 100 mg
• Chronic exposure through drinking water causes skin lesions, cancers and cardiovascular disease
• Arsine gas, formed when arsenides meet acid, is far more toxic still

The boiling point looks lower than the melting point because it is: grey arsenic sublimes at 887 K at ordinary pressure and only melts, at 1,090 K, under roughly 3.7 MPa. Known since antiquity as its sulfide ores; Albertus Magnus is usually credited with isolating the element around 1250.

Neighbours in the table

Group 15

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Arsenic sits in group 15 with nitrogen, phosphorus, antimony, bismuth, moscovium.