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.

Atomic properties

Standard atomic weight	74.9215 ±6·10^-6
Atomic mass	74.9215 u

Atomic radii

Radius (empirical)	119 pm
Radius (calculated)	114 pm
Covalent radius	119 ±4 pm
Van der Waals radius	185 pm

Atomic shell

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

Physical properties

Phase	solid
Density	gray: 5.73 g·cm⁻³ yellow: 1.97 g·cm⁻¹ black: 4.9 [4.7 … 5.1] g·cm⁻¹
Molar volume	1.295·10^-5 m³·mol⁻¹

Temperatures

Sublimation point	887 K
Triple point	1,090 K @ 3,628 kPa
Critical point	(1,673 K)
Transition temperature	2.7 K

Enthalpies

Melting enthalpy	27.7 kJ·mol^-1
Enthalpy of vaporization	32.4 kJ·mol^-1
Binding energy	302 kJ·mol^-1

Heat and conductivity

Specific heat capacity	328.5 J·kg⁻¹·K⁻¹
Thermal conductivity	50 W·m^-1·K^-1

Hardnesses

Mohs hardness	3.5
Brinell hardness	1,440 NM·m^-2

Elastic properties

Young’s modulus	8 GPa
Bulk modulus	22 GPa

Electrical properties

Electrical conductivity	3.03·10⁶ S·m^-1
Resistance	3.3·10^-7 Ωm

Magnetism

Magnetism	diamagnetic
Magnetic susceptibility	-5.5·10^-6 cm³·mol⁻¹

Optical properties

Refractive index	1.001

Chemical properties

Basicity	amphoteric
Oxidation state	-3, 3, 5
Standard potential	0.24 V (½As₂O₃ + 3H⁺ + 3e^- → As + 1½H₂O)

Electronegativity

Pauling scale	2.18
Sanderson scale	2.82
Allred-Rochow scale	2.2
Mulliken scale	2.26
Allen scale	2.211
Ghosh-Gupta scale	5.537 eV
Boyd-Edgecombe scale	2.21
Nagle scale	2.11
Pearson absolute negativity	5.3 eV

Other properties

Natural occurrence	primordial
Crystal structure	Rhombohedral
Goldschmidt Classification	chalcophile
Superconductor	with transition tempperature under special conditions
Price/kg	1.15 ±0.16 USD