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Bismuth
Bismuth (Bi, atomic number 83) is a silvery-pink post-transition metal in Group 15 of the periodic table, and the heaviest element with a stable (or near-stable) nuclide. With a density of 9.78 g/cm³ and a melting point of just 271.5 °C, bismuth is one of the densest common metals yet melts at a temperature accessible to simple electric furnaces. It adopts a rhombohedral crystal structure (A7 type) isostructural with arsenic and antimony, characterized by layered covalently bonded bilayers with weak van der Waals interlayer forces. This structure gives bismuth anomalous physical properties: it has the highest electrical resistivity of any metal (1.3 µΩ·m), the lowest thermal conductivity of any metal (7.97 W/m·K), and strong diamagnetism — the highest diamagnetic susceptibility of any element. Crucially, like water and gallium, bismuth expands on solidification (~3.3% volume increase), making it the basis of low-melting alloys that fill molds completely without shrinkage defects.
Bismuth occupies a unique position in the periodic table as effectively the last stable element, with all heavier nuclides being radioactive. The nominal "stable" isotope ²⁰⁹Bi was discovered in 2003 to be alpha-radioactive with a half-life of 1.9 × 10¹⁹ years — over a billion times the age of the universe — making it stable for all practical purposes. Bismuth's electronic structure features extremely light charge carriers with very long mean free paths, producing exceptional magnetoresistance, a very large Hall effect, and quantum oscillations (Shubnikov–de Haas and de Haas–van Alphen) that were historically central to the development of Fermi surface physics. Bi₂Te₃ and its alloys are the dominant thermoelectric materials for near-room-temperature applications, with figure of merit ZT ~1 at 300–450 K, underpinning commercial Peltier coolers and thermoelectric generators.
In applications, bismuth's combination of low toxicity, low melting point, expansion on solidification, high density, and unusual electronic properties gives it roles across pharmaceuticals, metallurgy, thermoelectrics, and radiation shielding. Bismuth subsalicylate is the active ingredient in widely used gastrointestinal medications (Pepto-Bismol), exploiting bismuth's bacteriostatic properties against H. pylori and other gut pathogens. Bismuth oxychloride (BiOCl) is the iridescent pigment responsible for the pearlescent appearance in cosmetics. Wood's metal, Rose's metal, and Field's metal — all bismuth-based low-melting alloys — are used in fire suppression sprinkler heads, fusible safety plugs, and prototype casting. Bismuth is increasingly used as a dense, non-toxic replacement for lead in ammunition, fishing weights, and radiation shielding.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 83 | Group 15 (pnictogen), Period 6; heaviest element with a stable (effectively) nuclide — all elements from Z = 84 onward have no stable isotopes |
| Atomic Mass | 208.980 u | Effectively monoisotopic — ²⁰⁹Bi is 100% natural abundance; its alpha decay half-life of 1.9 × 10¹⁹ yr renders it stable for all practical purposes |
| Density (20 °C) | 9.78 g/cm³ | ~3.6× denser than aluminum; denser than steel (7.87 g/cm³); high density exploited in radiation shielding and ballistic applications as a non-toxic lead substitute |
| Melting Point | 271.5 °C (544.6 K) | Low enough to melt on a standard hotplate; enables low-energy casting and easy alloying; the basis of bismuth-based fusible alloys for fire safety and prototype tooling |
| Boiling Point | 1,564 °C (1,837 K) | Wide liquid range of ~1,300 °C; bismuth vapour is moderately toxic — ventilation required during high-temperature processing |
| Thermal Conductivity | 7.97 W/m·K | Lowest thermal conductivity of any metal — a direct consequence of its semimetallic electronic structure; this property underpins its use in thermoelectric applications where low κ is essential for maintaining a thermal gradient |
| Electrical Resistivity | 1,300 nΩ·m (20 °C) | Highest electrical resistivity of any metal; ~50× greater than copper; semimetallic character with extremely light, long-mean-free-path charge carriers giving exceptional magnetoresistance |
| Crystal Structure | Rhombohedral (A7 type) | Isostructural with arsenic and antimony; layered covalently bonded bilayers held by weak van der Waals forces; cleaves easily along (001); highly anisotropic electrical and thermal properties in single-crystal form |
| Volume Change on Solidification | +3.3% | Bismuth expands on freezing, like water and gallium; exploited in low-melting fusible alloys (Wood's metal, Rose's metal) that reproduce mold detail without shrinkage cavities |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Hardness | Mohs 2.25; ~16 HV | Very soft — scratched by a fingernail; brittle and easily fractured along cleavage planes; cannot be rolled or drawn without fracture at room temperature |
| Elastic (Young's) Modulus | 32 GPa | Very low stiffness — comparable to lead (16 GPa) and tin (47 GPa); highly anisotropic in single-crystal form; relevant for solder joint compliance calculations |
| Poisson's Ratio | 0.33 | Typical for metallic systems; relevant to thermal stress calculations in low-melting alloy applications |
| Fracture Behavior | Brittle (transgranular) | No measurable room-temperature ductility; cleaves along rhombohedral planes; shatters on impact rather than deforming plastically |
Thermal & Environmental Properties
| Property | Value | Notes |
|---|---|---|
| Corrosion Resistance | Good in dry air; moderate in humidity | Forms a thin Bi₂O₃ passivation layer in air; stable in dilute H₂SO₄ and HCl at room temperature; dissolved by concentrated HNO₃ and hot H₂SO₄; resistant to alkalis |
| Oxidation States | +3 (primary), +5 | Bi³⁺ is the dominant state; Bi⁵⁺ is a strong oxidizer, appearing in NaBiO₃ and BiF₅; the +3/+5 redox couple is exploited in selective oxidation catalysis |
| Diamagnetism | –1.66 × 10⁻⁴ (volume susceptibility) | Strongest diamagnetic element; can be partially levitated in strong magnetic field gradients; used in diamagnetic levitation demonstrations and magnetometer calibration |
| Toxicology | Low toxicity (regulatory basis for lead replacement) | Bismuth and most of its compounds have very low mammalian toxicity; bismuth subsalicylate is an approved pharmaceutical active ingredient; key driver of bismuth's adoption as a non-toxic substitute for lead in shot, solder, and pigments |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Surface Oxide | Bi₂O₃ (yellow, monoclinic α-form at RT) | Multiple polymorphs; α-Bi₂O₃ is stable at room temperature; δ-Bi₂O₃ (high temperature) is a superionic conductor with exceptionally high O²⁻ conductivity — of interest for solid oxide fuel cell electrolytes |
| Acid Resistance | Resistant to dilute H₂SO₄, HCl; attacked by HNO₃ | Dissolves readily in concentrated HNO₃ forming Bi(NO₃)₃; resistant to dilute non-oxidizing acids; resistant to alkalis — does not exhibit amphoteric behavior unlike As and Sb |
| Thermoelectric Properties | Bi₂Te₃: ZT ~1 at 300–450 K | Bismuth telluride and its alloys (Bi₂Te₃/Sb₂Te₃/Bi₂Se₃) are the dominant near-room-temperature thermoelectric materials; Seebeck coefficient –200 to +300 µV/K depending on composition and doping |
| Topological Properties | Bi and Bi₁₋ₓSbₓ are topological semimetals | Elemental Bi and Bi-Sb alloys are archetypes of topological semimetals and topological insulators; the surface states of Bi(111) were among the first experimentally confirmed topological surface states, of current interest for quantum computing platforms |
| Identifier | Value |
|---|---|
| Symbol | Bi |
| Atomic Number | 83 |
| CAS Number | 7440-69-9 |
| UN Number | UN3089 (powder) |
| EINECS Number | 231-177-4 |
| Isotope | Type | Notes |
|---|---|---|
| ²⁰⁹Bi | Stable* | 100% natural abundance; alpha-radioactive with t½ = 1.9 × 10¹⁹ yr — over a billion times the age of the universe; treated as stable for all practical purposes; I = 9/2, NMR-active |
| ²⁰⁷Bi | Radioactive | t½ = 31.55 yr (electron capture); gamma emitter (0.570, 1.063, 1.770 MeV); used as a long-lived gamma calibration source in detector energy calibration |
| ²⁰⁶Bi | Radioactive | t½ = 6.24 days (electron capture); produced by proton irradiation of lead; multiple gamma lines used in detector calibration and tracer studies |
| ²¹³Bi | Radioactive | t½ = 45.6 min (β⁻/α); alpha emitter of current interest in targeted alpha therapy (TAT) for cancer treatment — the ²²⁵Ac/²¹³Bi generator system is in clinical trials for leukaemia and other malignancies |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Thermoelectric Materials Research | High-purity ingot, sputtering targets, MBE sources | Bismuth telluride (Bi₂Te₃) and its alloys with Sb₂Te₃ and Bi₂Se₃ are the benchmark near-room-temperature thermoelectric materials, with ZT ~1 at 300–450 K. Research focuses on nanostructuring to suppress thermal conductivity while maintaining electrical performance, and on p-type/n-type leg optimization for Peltier coolers and thermoelectric generators. |
| Topological Insulator & Semimetal Research | Single crystals, thin films, MBE-grown heterostructures | Bi and Bi₁₋ₓSbₓ alloys are archetypes of topological semimetals and topological insulators, hosting topologically protected surface states of interest for quantum computing (Majorana fermion platforms) and low-dissipation spintronics. The Bi(111) surface was among the first experimentally confirmed topological surface state systems. |
| Low-Melting Alloy Research | Ingot, shot, granules | Bismuth-based fusible alloys — Wood's metal (Bi 50%, Pb 26.7%, Sn 13.3%, Cd 10%), Rose's metal (Bi 50%, Pb 28%, Sn 22%), and Field's metal (Bi 32.5%, In 51%, Sn 16.5%) — are studied for precision casting, prototype tooling, and as surrogate materials for low-melting nuclear fuel rod research where exact volume behavior on solidification matters. |
| Targeted Alpha Therapy (TAT) | High-purity Bi salts, generator targets | ²¹³Bi, produced from ²²⁵Ac generator systems, is a leading alpha-emitting radioisotope for targeted cancer therapy — its 45.6-minute half-life and 8.4 MeV alpha energy are well-matched to tumor cell killing range. Bi-213 labeled antibodies and peptides are in Phase I/II clinical trials for acute myeloid leukaemia and neuroendocrine tumors. |
| Magnetoresistance & Fermi Surface Studies | Single crystals, oriented wafers | Bismuth's semimetallic electronic structure — with an extraordinarily small Fermi surface, very light effective masses, and long carrier mean free paths — produces huge magnetoresistance effects and clear quantum oscillations (de Haas–van Alphen and Shubnikov–de Haas) that were historically central to the development of Fermi surface theory and are now studied in the context of Weyl and Dirac semimetals. |
| Selective Oxidation Catalysis | Bi₂O₃ powder, Bi-Mo mixed oxide targets | Bismuth molybdate (Bi₂MoO₆ and related phases) is the industrial catalyst for the selective oxidation of propylene to acrolein — the key step in acrylic acid production. Bi's ability to activate C–H bonds selectively while suppressing over-oxidation makes it central to several industrial oxidation processes. |
Industrial & Commercial Applications
| Sector | Form / Compound Used | Description |
|---|---|---|
| Pharmaceuticals | Bismuth subsalicylate, bismuth subcitrate | Bismuth subsalicylate (Pepto-Bismol) is an FDA-approved active ingredient for treating nausea, heartburn, and diarrhea; it acts through bacteriostatic effects on H. pylori and other gut pathogens, and by coating the gastrointestinal mucosa. Bismuth subcitrate is a key component of quadruple therapy for H. pylori eradication. |
| Cosmetics & Pigments | Bismuth oxychloride (BiOCl) | Bismuth oxychloride produces a brilliant iridescent pearlescent effect through platelet-shaped crystals that reflect and diffract light — the standard pigment in eyeshadow, nail polish, and foundation. It is valued for its skin-compatibility, chemical inertness, and non-toxicity relative to lead-based pearl pigments it replaced. |
| Non-Toxic Shot & Ammunition | Bi-Sn alloy shot (97% Bi, 3% Sn) | Bismuth-tin shot is the leading non-toxic alternative to lead shot for waterfowl hunting, mandated by law in many countries following bans on lead shot near wetlands. Density (~9.6 g/cm³) closely matches lead (11.3 g/cm³), minimizing performance degradation, while the non-toxic profile eliminates environmental lead accumulation and waterfowl poisoning. |
| Fire Suppression & Safety Devices | Wood's metal, Rose's metal, low-melting alloys | Bismuth-based fusible alloys melt at precisely defined temperatures (47–100 °C depending on composition) and are used as actuating elements in automatic fire sprinkler heads, boiler safety plugs, and temperature-sensitive circuit breakers. Their expansion on solidification ensures bubble-free, void-free plug formation critical for reliable actuation. |
| Radiation Shielding | Bi sheet, Bi-loaded polymers, BiO compounds | Bismuth's high atomic number (Z = 83) and density give it X-ray and gamma attenuation comparable to lead, while its low toxicity enables use in medical radiation shielding garments, aprons, and thyroid collars as a lead-free alternative. Bi₂O₃-loaded rubber and polymer composites are used in radiation protection in medical imaging suites. |
| Metallurgical Additives | Bi ingot, master alloys | Bismuth is added at 0.1–0.5 wt% to free-machining steels and aluminum alloys to improve machinability by acting as a chip-breaker — replacing the more toxic lead additions in lead-free free-machining brass and steel grades. Bi also improves the castability of tin-based solders by reducing surface tension. |
| Purity | Main Use |
|---|---|
| 97% | General industrial use, pigment and alloy production — suitable for bismuth oxychloride cosmetic pigment synthesis, low-melting fusible alloy production, and free-machining alloy additives where trace impurities are acceptable |
| 99.5% | Thermoelectric prototypes and low-temperature alloys — appropriate for laboratory synthesis of Bi₂Te₃ thermoelectric precursors, Rose's metal and Wood's metal formulations, and pharmaceutical-grade bismuth salt synthesis where controlled impurity profiles are required |
| 99.97% | Electronics and advanced alloy research — used in lead-free solder development, BiSb and BiSbTe thin-film deposition targets, and precision low-melting alloy casting where impurity segregation must be minimized |
| 99.997% | Sensitive detectors and semiconductors — the standard purity for Bi₂Te₃ thermoelectric module production, radiation detector crystal growth, and compound semiconductor synthesis where sub-ppm impurities are required to control carrier concentration and Seebeck coefficient |
| 99.999% | High-precision instrumentation — used in single-crystal growth for Fermi surface studies, topological insulator research, and magnetoresistance measurements where ppb-level metallic impurities would introduce extrinsic scattering and mask intrinsic electronic behavior |
| 99.9999% | Cutting-edge materials science and nanotechnology — the highest available purity for quantum transport experiments, topological surface state ARPES studies, and MBE source material for Bi-based heterostructures where any extrinsic impurity would obscure quantum phenomena |
| Synonym / Alternative Name | Context |
|---|---|
| Bi | Chemical symbol |
| Bismuth metal | Standard commercial and regulatory designation for the elemental form |
| Wismut | German name; origin uncertain — possibly from Arabic bi ismid (having the properties of antimony) or from the German Weisse Masse (white mass); source of the modern name in most European languages |
| Bismuto | Spanish and Italian language equivalent |
| Bismuth cristallisé | French term for the characteristic hopper-crystal form produced by slow solidification, displaying iridescent oxide surface coloring |
| Elemental bismuth | General scientific term distinguishing the pure metal from bismuth compounds (Bi₂Te₃, BiOCl, Bi₂O₃, etc.) |