Arsenic is a chemical element with the symbol As and atomic number 33. It is a element in the nitrogen group (Group 15 [Va] of the periodic table), existing in both gray and yellow crystalline forms. The primary use of As is in alloys of lead for example, in car batteries and ammunition).
It is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the second most commonly used semiconductor after doped silicon. Its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. These applications are declining due to the toxicity of As and its compounds.
Physical and Chemical Characteristics of Arsenic
- It allotropes are gray, yellow, and black arsenic, with gray being the most common. Gray As (α-As, space group R3m No. 166) adopts a double-layered structure consisting of many interlocked, ruffled, six-membered rings.
- Because of weak bonding between the layers, gray As is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next.
- This relatively close packing leads to a high density of 5.73 g/cm3. Gray As is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized. Gray As is also the most stable form. Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus.
- Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecular, is the most volatile, least dense, and most toxic.
- Solid yellow As is produced by rapid cooling of arsenic vapor, As4. It is rapidly transformed into gray As by light. The yellow form has a density of 1.97 g/cm3.
- Black arsenic is similar in structure to black phosphorus. Black arsenic can also be formed by cooling vapor at around 100–220 °C. It is glassy and brittle. It is also a poor electrical conductor.
- It has a similar electronegativity and ionization energies to its lighter congener phosphorus and as such readily forms covalent molecules with most of the nonmetals.
- Though stable in dry air, arsenic forms a golden-bronze tarnish upon exposure to humidity which eventually becomes a black surface layer.
- It burns in oxygen to form arsenic trioxide and arsenic pentoxide, which have the same structure as the more well-known phosphorus compounds, and in fluorine to give arsenic pentafluoride.