All practice pages/Interatomic theory/Elements
Element 33, a metalloid
A metalloid infamous as a poison, present in groundwater around the world, and also a modern leukemia drug.
Period 4, group 15. The other metalloids are tinted, and every cell opens its own article.
Arsenic has 33 electrons, one for every proton, so the atom carries no charge. Below are the same 33 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³
Short form: [Ar] 3d¹⁰ 4s² 4p³, where [Ar] stands for the 18 electrons argon already has.
That list runs by shell. In the order the subshells fill it reads [Ar] 4s² 3d¹⁰ 4p³, because a subshell fills by its energy, not by its shell number.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.
Arsenic sits in period 4 and has 4 shells. The period number counts the shells.
The outer shell holds 5 electrons. Those are the ones arsenic can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 15 means 5.
Five valence electrons, like nitrogen and phosphorus. Arsenic mostly shares electrons in covalent bonds, taking the +3 or +5 state with oxygen.
Occurs in many minerals, especially arsenopyrite (FeAsS), and as the colorful sulfides realgar and orpiment. Most is recovered while smelting copper and gold ores. Arsenic dissolved from rock contaminates well water in Bangladesh, India and other regions.
Arsenic compounds were known in the ancient world as pigments and poisons, and Albertus Magnus is credited with isolating the element around 1250. The name comes through the Greek arsenikon from the Persian zarnikh, yellow orpiment.
Gallium arsenide semiconductors, and hardening the lead in car batteries and ammunition. It was once widely used in wood preservatives, pesticides and green pigments such as Paris green. Arsenic trioxide is an effective treatment for one type of leukemia, acute promyelocytic leukemia.
Arsenic and its compounds are toxic and cause cancer. It was so common a murder weapon that it was nicknamed inheritance powder, until the Marsh test of 1836 made it easy to detect.