33AsArsenic

Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.

The primary use of arsenic is in alloys of lead (for example, in car batteries and ammunition). Arsenic is a common n-type dopant in semiconductor electronic devices. It is also a component of the III–V compound semiconductor gallium arsenide. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. These applications are declining with the increasing recognition of the toxicity of arsenic and its compounds. A few species of bacteria are able to use arsenic compounds as respiratory metabolites. Trace quantities of arsenic are an essential dietary element in rats, hamsters, goats, chickens, and presumably other species. A role in human metabolism is not known. However, arsenic poisoning occurs in multicellular life if quantities are larger than needed. Arsenic contamination of groundwater is a problem that affects millions of people across the world.

The United States' Environmental Protection Agency states that all forms of arsenic are a serious risk to human health. The United States' Agency for Toxic Substances and Disease Registry ranked arsenic as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites. Arsenic is classified as a Group-A carcinogen.

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Atomic properties

Standard atomic weight74.9215 ±6·10-6
Atomic mass74.9215 u

Atomic radii

Radius (empirical)119 pm
Radius (calculated)114 pm
Covalent radius119 ±4 pm
Van der Waals radius185 pm

Atomic shell

Electron configurationAr 3d10 4s2 4p3
Ionization energy(1st) 9.78855 eV
(2nd) 18.5892 eV
(3rd) 28.349 eV
(4th) 50.15 eV
(5th) 62.77 eV
Shell model
Electrons
k-shell:2
l-shell:8
m-shell:18
n-shell:5

Physical properties

Phasesolid
Densitygray: 5.73 g·cm−3
yellow: 1.97 g·cm−1
black: 4.9 [4.7 … 5.1] g·cm−1
Molar volume1.295·10-5 m3·mol−1

Temperatures

Sublimation point887 K
Triple point1,090 K @ 3,628 kPa
Critical point(1,673 K)
Transition temperature2.7 K

Enthalpies

Melting enthalpy27.7 kJ·mol-1
Enthalpy of vaporization32.4 kJ·mol-1
Binding energy302 kJ·mol-1

Heat and conductivity

Specific heat capacity328.5 J·kg−1·K−1
Thermal conductivity50 W·m-1·K-1

Hardnesses

Mohs hardness3.5
Brinell hardness1,440 NM·m-2

Elastic properties

Young’s modulus8 GPa
Bulk modulus22 GPa

Electrical properties

Electrical conductivity3.03·106 S·m-1
Resistance3.3·10-7 Ωm

Magnetism

Magnetismdiamagnetic
Magnetic susceptibility-5.5·10-6 cm3·mol−1

Optical properties

Refractive index1.001

Chemical properties

Basicityamphoteric
Oxidation state-3, 3, 5
Standard potential0.24 V (½As2O3 + 3H+ + 3e- → As + 1½H2O)

Electronegativity

Pauling scale2.18
Sanderson scale2.82
Allred-Rochow scale2.2
Mulliken scale2.26
Allen scale2.211
Ghosh-Gupta scale5.537 eV
Boyd-Edgecombe scale2.21
Nagle scale2.11
Pearson absolute negativity5.3 eV

Other properties

Natural occurrenceprimordial
Crystal structureRhombohedral
Goldschmidt Classificationchalcophile
Superconductorwith transition tempperature under special conditions
Price/kg1.15 ±0.16 USD

Natural abundances

satellite_alt
Universe
8 ppb ≈ 8.04·1010 M☉
sunny
destruction
Meteorite
1,800 ppb ≈ 1.79 g
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Earth’s crust
2,100 ppb ≈ 58,100 Mt
water_drop
Oceans
2.3 ppb ≈ 3.15 Gt
waves
Flowing water
1 ppb ≈ 16 kt
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Human body
50 ppb ≈ 3.5 mg