The electron configuration of arsenic is [Ar] 4s2 3d10 4p3. It exists in many forms. Allotropes of arsenic are yellow, black and gray, according to About.com. The gray allotrope is the most common form of elemental arsenic and a conductor of electricity. In the other colors, arsenic is a poor conductor of electricity.
The electron configuration of arsenic allows it to combine in several different valances, including -3, 0, +3 and +5. It combines with other elements, including hydrogen, oxygen, the halogens and sulfur. Many of these compounds are extremely poisonous. Arsenic (III) oxide of white arsenic is one of the most well-known. In dilute concentrations, too weak to be poisonous, arsenic is a carcinogen, according to Reference.com.
The synthetic arsenates, Paris Green (copper(II) acetoarsenite), calcium arsenate and lead hydrogen arsenate were once common as agricultural insecticides, until the discovery of D.D.T. in 1942. Other uses include metal alloys. Arsenic is a common chemical for use in the semiconductor industry.
Before modern antibiotics, arsenic compounds were common for the treatment of Syphilis and trypanosomiasis. About.com indicates Victorian women consumed mixtures of vinegar, chalk and arsenic as a tonic to lighten the completion. Research shows radioactive arsenic-74 as a good candidate for a positron emitter for PET scans in locating cancerous tumors.