We continue to review some of the most important materials in heat treatment and metallurgy.
Bismuth (chemical symbol: Bi)
Bismuth is a hard, brittle metal that grows in staircase-shaped crystals (Fig. 1) due to a greater growth rate on its outside edges than on the inside. It is a dull, silvery-white color but becomes slightly pink after surface oxidation occurs. Bismuth is one of the few elements whose solid state is less dense than its liquid state. Therefore, it expands when it freezes. Other elements that share this attribute are silicon, gallium, germanium, antimony and plutonium. This characteristic makes it valuable as an alloying element in equal parts with lead to compensate for the contraction of lead when it solidifies. It is also used in typesetting because it grows to fill the typeset mold when solidifying, providing a sharp-edged text for printing.
Bismuth has been used since ancient times and was for centuries mistaken for lead and tin, both neighboring elements on the periodic table. French chemist Claude Francois Geoffroy first identified bismuth to be a distinct element in 1753, becoming the official discoverer of the element. He died later that year at the age of 24. Geoffroy was part of a famed lineage of chemists. He and his brother Etienne were the fifth generation of a renowned family that had entertained various scientists, such as famed astronomer J.D. Cassini.
Bismuth is commonly alloyed with lead, tin, iron or cadmium to form low-melting alloys used in automatic fire-sprinkler systems, fire-detection systems and electrical fuses. Bismuth has an unusually low toxicity among the heavy metals and has been considered as a replacement for lead in water pipes. About one-third of the bismuth produced is used as a safer replacement for lead in ceramic glazes, fishing sinkers, food-processing equipment, free-machining brasses for plumbing applications, lubricating greases and shot for waterfowl hunting.
Bismuth compounds have many uses in medicine. Bismuth subsalicylate is the active ingredient in Pepto Bismol (aka pink bismuth) and Kaopectate to treat upset stomachs. Interestingly, it is still not understood exactly how the medicine works. Other bismuth compounds are used to treat peptic ulcers, eye infections and flatulence. Bismuth compounds are also used as a pigment in eye shadow, hair spray and nail polish.
Here are a few important facts about bismuth.
- Atomic number: 83
- Atomic weight: 208.98040
- Melting point: 544.55 K (271.40°C or 520.52°F)
- Boiling point: 1837 K (1564°C or 2847°F)
- Density: 9.807 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: 15
- Group name: Pnictogen