Arsenic (As)

Stable Isotopes of Arsenic

Isotope Z(p) N(n) Atomic Mass Natural Abundance Nuclear Spin
As-75  33  42  74.921597 100.00%  3/2- 



Arsenic compounds were mined by the early Chinese, Greek and Egyptian civilizations. It is believed that Albertus Magnus obtained the element in 1250 AD by heating soap together with orpiment (arsenic trisulfide, used as a dye or pigment). The element takes its name from the Greek word arsenikon, meaning "yellow orpiment."

Arsenic is a silver-gray, brittle, crystalline solid that is stable in dry air. Exposure to moist air tarnishes its surface to a golden bronze color, which converts to a black oxide on further exposure. Three allotropes are known: the α-metallic form, the black amorphous vitreous solid known as β-arsenic, and a yellow allotrope. It is insoluble in water as well as in caustic and nonoxidizing acids. Arsenic is attacked by HCl in the presence of oxidant, and it reacts with nitric acid. Arsenic sublimes at 613 ºC when heated at normal atmospheric pressure. It has densities of 5.72 g/cc (in the β-metallic form) and 4.70 g/cm (in the β-amorphous form.

The major uses of arsenic are in metallurgy — primarily as an additive to lead, copper, brass and many lead-based bearing alloys — to improve their mechanical and thermal properties. The addition of very small quantities of arsenic to copper enhances copper's corrosion resistance. It also prevents cracking in brass. 

Elemental arsenic is much less toxic than its soluble compounds. Only its uncommon yellow allotrope is highly toxic. Inhalation of its metal dusts can cause ulceration of the nasal septum. Ingestion may produce systemic skin and gastrointestinal effects in humans. Arsenic and its compounds are human carcinogens, producing liver tumors.

Properties of Arsenic

Name Arsenic 
Symbol As 
Atomic number 33 
Atomic weight 74.9216 
Standard state Solid at 298 °K 
CAS Registry ID 7440-38-2 
Group in periodic table 15 
Group name Pnictogen 
Period in periodic table
Block in periodic table p-block 
Color Metallic gray 
Classification Semi-metallic 
Melting point 817 °C at 28 atm
Boiling point 613 °C subl.
Thermal conductivity 50 W/(m·K)
Electrical resistivity 33.3 µΩ·cm at 20 °C 
Electronegativity 2.18
Specific heat 328 J/(kg·K)
Heat of vaporization

32.4 kJ·mol-1

Heat of fusion

27.7 kJ·mol-1

Density of liquid 5.22 g/cm-3 at 817 °C 
Density of solid 5.727 g/cm
Covalent radius  As3+ :1.21 Å 
Electron configuration  [Ar]4s23d104p3 
Principal oxidation states  -3, 0, +3, +5 

Isotope Supplier: ISOFLEX logo