Introduction to Bismuth
Bismuth (symbol Bi, atomic number 83) is a heavy, silvery-white metal with a characteristic pinkish tinge. It belongs to Group 15 of the periodic table, along with nitrogen, phosphorus, arsenic, and antimony. Despite its metallic appearance, bismuth possesses some unique properties that distinguish it from many other heavy metals. It is known for its low melting point and its beautiful iridescent oxide layer when formed under certain conditions.
General Properties
Bismuth is diamagnetic and has a high electrical resistance when in its solid state. It is also one of the few elements that expands upon solidification, a property it shares with water and silicon. It finds applications in low-melting alloys, cosmetics, and pharmaceuticals. In India, bismuth compounds are used in various over-the-counter medications for stomach ailments.
Chemical Reactivity of Bismuth
Bismuth is considered to have relatively low chemical reactivity compared to many other metals. Its reactivity is influenced by temperature and the presence of oxidizing agents.
Reaction with Air
Bismuth metal is stable in dry air at room temperature, meaning it does not readily react with oxygen or nitrogen under these conditions. However, when exposed to moist air over time, a thin, protective layer of bismuth(III) oxide (Bi₂O₃) forms on its surface, giving it a duller appearance.
Upon heating in air or oxygen, bismuth burns with a blue flame, forming bismuth(III) oxide: 4 Bi(s) + 3 O₂(g) → 2 Bi₂O₃(s)
Reaction with Water
Bismuth does not react with water or steam at room temperature. It is resistant to corrosion by neutral water. At elevated temperatures, specifically at red heat, bismuth reacts with steam to produce bismuth(III) oxide and hydrogen gas: 2 Bi(s) + 3 H₂O(g) → Bi₂O₃(s) + 3 H₂(g)
Reaction with Acids
Bismuth does not react with non-oxidizing acids, such as dilute hydrochloric acid (HCl) or dilute sulfuric acid (H₂SO₄), at room temperature. This indicates its position below hydrogen in the electrochemical series.
However, bismuth reacts with strong oxidizing acids. For instance, it dissolves in hot concentrated sulfuric acid and in nitric acid. When reacting with concentrated nitric acid, it forms bismuth(III) nitrate, releasing nitrogen monoxide or nitrogen dioxide, depending on the acid concentration and temperature: Bi(s) + 4 HNO₃(conc) → Bi(NO₃)₃(aq) + NO(g) + 2 H₂O(l)
Bismuth does not react with alkalis (bases).
Safety Profile
Toxicity
Historically, bismuth compounds were often confused with lead compounds due to similar appearance and some similar uses. However, bismuth is considerably less toxic than lead and many other heavy metals. While large doses of soluble bismuth compounds can be toxic, elemental bismuth and many of its compounds are used safely in medicine and cosmetics. For example, bismuth subsalicylate is an active ingredient in some common antacids and anti-diarrhea medications widely available in India.
Radioactivity
For many decades, bismuth-209 was considered the heaviest stable isotope. However, in 2003, it was discovered to be an alpha emitter with an extremely long half-life of 1.9 × 10¹⁹ years. This half-life is billions of times longer than the age of the universe, rendering it practically stable for all terrestrial and chemical purposes. Therefore, for all practical considerations in chemistry and common applications, bismuth is treated as a stable, non-radioactive element.
Flammability
Bulk metallic bismuth is not flammable. It does not ignite or sustain a flame under normal conditions. Like many metals, finely powdered bismuth can be combustible under specific, high-energy ignition conditions, but this is not typical behavior for the solid metal.
Notable Chemical Reaction
One significant chemical reaction involving bismuth is its dissolution in nitric acid to form bismuth(III) nitrate. This reaction is fundamental because bismuth(III) nitrate is a key precursor for synthesizing a wide range of other bismuth compounds, including pharmaceuticals, pigments, and catalysts.
Bi(s) + 4 HNO₃(conc) → Bi(NO₃)₃(aq) + NO(g) + 2 H₂O(l)
This reaction demonstrates bismuth’s metallic character when encountering a strong oxidizing agent, leading to the formation of a soluble salt.