We continue to review some of the most important materials in heat treatment and metallurgy.
Arsenic (chemical symbol: As)
Arsenic is a crystalline metalloid (semi-metal), which is a material that sometimes behaves as a semiconductor and other times a conductor similar to the elements of boron, silicon and germanium. Elemental arsenic can be found in nature (Fig. 1) and has three common forms: gray, yellow and black arsenic. Gray arsenic is the most stable form and yellow arsenic the most unstable. Arsenic is probably best known to the public as a poison (arsenic trioxide), which is an odorless and tasteless powder resembling the appearance of wheat flour that can kill a human in 2-24 hours if ingested.
Arsenic was employed during the Bronze Age as an alloy-strengthener, a process that continued later by the ancient Chinese, Greeks and Egyptians. Even today, arsenic is still used as an alloying agent for copper and brass. The addition of arsenic to brass (a copper-zinc alloy) prevents a phenomenon known as dezincification, where zinc is extracted from brass in a type of corrosion.
Arsenic-lead alloys have many uses, such as in car batteries and ammunition, and only a very small amount of arsenic strengthens the lead significantly. Arsenic is a common n-type dopant in semiconductors (Fig. 2), and the compound gallium arsenide is the second most common semiconductor after doped silicon. Electrical components made from gallium arsenide are much faster, albeit more expensive, than their more common silicon counterparts.
Despite arsenic’s well-known toxicity, there is a bacterium called GFAJ-1 that feeds on the element. When starved of the phosphorus it normally requires, the bacterium instead integrates arsenic into its DNA and continues reproducing normally. There are actually harmless, trace amounts of arsenic found in humans and other animals.
One of the most unique traits of arsenic is that it sublimates directly from a solid into a gas when heated to 614°C (1137°F) at atmospheric pressure. It will only form a liquid at increased atmospheric pressure. At 28 times standard atmospheric pressure, arsenic melts at a temperature of 817°C (1503°F). For this reason, the boiling point of arsenic is shown as lower than the melting point on most data tables.
Here are a few important facts about arsenic.
- Atomic number: 33
- Atomic weight: 74.921595
- Melting point: 1090 K (817°C or 1503°F)
- Boiling Point: 887 K (614°C or 1137°F)
- Density: 5.776 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Semi-metal
- Period number: 4
- Group number: 15
- Group name: Pnictogen