Chemical element · reference page

33As

Arsenic

Arsenic is element 33 (As), located in period 4, group 15 and the p block. At room temperature its reference phase is solid. Its electron configuration is [Ar] 3d10 4s2 4p3.

Atomic number33

Standard atomic weight74.921595(6)

Group15

Period4

Blockp

Electron configuration[Ar] 3d10 4s2 4p3

Electronic structure

What controls its chemistry

Group 15 atoms have five valence electrons. They can form three covalent bonds, participate in multiple bonding, or adopt several oxidation states depending on the element and its bonding partners.

The configuration [Ar] 3d10 4s2 4p3 places arsenic in this family, but the chemistry changes markedly from non-metallic nitrogen and phosphorus to the heavier, more metallic members.

Chemical forms

Compounds, ions and materials

Common families include hydrides, oxides, oxoacids or oxoanions, halides and compounds with metals. Oxidation states from −3 to positive values occur across the group, with the accessible range changing strongly from nitrogen to bismuth.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 5.727 g/cm³. Where a phase-transition value is not listed, the behaviour may be poorly defined under ordinary pressure, depend strongly on allotrope, or lack a reliable reference value. Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where arsenic is found and used

In practice, arsenic is encountered in semiconductors, alloys and wood preservation.

In practice

Examples of real uses

Semiconductors

Compounds or doped forms of arsenic are used to control electrical conduction in semiconductor devices.

Alloys

Arsenic is added to metals to change strength, corrosion resistance, melting behaviour or high-temperature performance.

Wood preservation

Some arsenic compounds were used historically to protect timber; many uses are now restricted because of toxicity concerns.

Periodic position

What its neighbours tell us

Arsenic is in period 4 and group 15. In the neighbourhood shown here, it lies between germanium and selenium in the same period; phosphorus is above it and antimony below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationmetalloid

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 5

Density5.727 g/cm³

Melting point—

Boiling point—

Pauling electronegativity2.18

First ionisation energy944kJ/mol

Electron affinity77.65 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

Use of arsenic is recorded from the Bronze Age. Its early use predates modern chemistry, so use of the material, recognition of the element and the later atomic-number definition are separate stages.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 74.921595(6). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Arsenic has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

Many arsenic compounds are toxic and carcinogenic. Chemical form and exposure route strongly affect risk.