Platinum
Platinum is a chemical element with the symbol Pt and atomic number 78, classified as a dense, malleable, ductile, corrosion-resistant, silvery-white transition metal in group 10 of the periodic table.[1][2] It belongs to the platinum-group metals (PGEs), which include platinum, palladium, rhodium, ruthenium, iridium, and osmium, sharing similar physical and chemical properties such as high melting points and catalytic activity.[3] With a melting point of 1,768 °C, a boiling point of 3,825 °C, and a density of 21.45 g/cm³ at room temperature, platinum is one of the densest elements and exhibits exceptional stability against oxidation and chemical attack, making it highly valued for both aesthetic and industrial applications.[2][4] Native platinum and its alloys have been known since ancient times, with pre-Columbian South American civilizations using platinum-rich gold artifacts as early as 600 B.C., though they lacked the technology to work pure platinum.[5] The element was formally discovered in the 18th century when Spanish explorer Antonio de Ulloa encountered it in Colombia in 1735, describing it as "platina del Pinto" (little silver of the Pinto River), and independently identified by English merchant Charles Wood in 1741 from similar South American sources.[1] Platinum occurs naturally in native form, as alloys with other PGEs like iridium, or in rare minerals such as sperrylite (PtAs₂); it is primarily extracted from ultramafic igneous rocks and placer deposits, with global production dominated by South Africa (about 70%) and Russia (about 10%), accounting for about 80% of the world's supply as of 2024.[6][7] Platinum's unique properties— including resistance to corrosion, stable electrical conductivity, and catalytic efficiency— render it indispensable in diverse sectors.[5] In jewelry, it comprises about 28% of demand as of 2024 due to its luster, hypoallergenic nature, and durability, often alloyed with other metals for strength.[8] Industrially, it serves as a catalyst in automotive catalytic converters to reduce emissions by converting harmful gases like carbon monoxide and nitrogen oxides, and in chemical processes such as the oxidation of ammonia to nitric acid.[9] In electronics, platinum is used for hard disk drives, electrodes, and thermocouples owing to its reliable performance at high temperatures.[5] Medically, platinum compounds like cisplatin are key in chemotherapy for treating cancers such as testicular and ovarian tumors, while pure platinum features in pacemakers, stents, and neural electrodes for its biocompatibility.[7] Annual global mine production was about 180 metric tons in 2024, with recycling from autocatalysts recovering an additional 20-25%, and recent market deficits underscoring its economic importance as a precious metal.[10]Physical and Chemical Properties
Physical Properties
Platinum is a silvery-white transition metal with the chemical symbol Pt and atomic number 78, positioned in group 10 and period 6 of the periodic table.[4] It exhibits a lustrous appearance and is renowned for its exceptional density, measuring 21.45 g/cm³ at 20°C, which ranks it as the densest among common metals aside from osmium and iridium.[4][11] This high density contributes to its weighty feel and stability in applications requiring durability. The metal has a high melting point of 1768.3°C and a boiling point of 3825°C, reflecting strong metallic bonding that allows it to withstand extreme temperatures without phase change.[11] Platinum's crystal structure is face-centered cubic (FCC), with a lattice constant of 3.9239 Å, which underpins its close-packed atomic arrangement and isotropic properties.[12] Platinum demonstrates remarkable malleability and ductility, enabling it to be drawn into wires as thin as 0.0025 mm or hammered into sheets 0.1 μm thick, properties that facilitate its fabrication into intricate forms.[2] Its electrical resistivity is 10.6 μΩ·cm at 20°C, making it a reliable conductor in precision instruments, while its thermal conductivity stands at 71.6 W/(m·K).[4][13] Additionally, the specific heat capacity is 0.133 J/(g·K), and the coefficient of thermal expansion is 8.8 × 10⁻⁶ /K, indicating moderate heat absorption and low dimensional change with temperature variations.[14][15]| Property | Value | Conditions |
|---|---|---|
| Density | 21.45 g/cm³ | 20°C |
| Melting Point | 1768.3°C | - |
| Boiling Point | 3825°C | - |
| Electrical Resistivity | 10.6 μΩ·cm | 20°C |
| Thermal Conductivity | 71.6 W/(m·K) | - |
| Specific Heat Capacity | 0.133 J/(g·K) | - |
| Coefficient of Thermal Expansion | 8.8 × 10⁻⁶ /K | - |
Chemical Properties
Platinum is classified as a noble metal, characterized by its exceptionally low reactivity and high resistance to corrosion. It does not oxidize in air at any temperature, maintaining its metallic form even under prolonged exposure to oxygen.[16] This inertness extends to most acids, where platinum remains unattacked by single mineral acids such as hydrochloric or nitric acid, but it dissolves in aqua regia, a 3:1 mixture of concentrated hydrochloric and nitric acids that generates nascent chlorine to facilitate oxidation.[4][17] The element exhibits common oxidation states of 0, +2, and +4 in its compounds. In the +2 state, platinum typically adopts a square planar coordination geometry, while the +4 state features octahedral coordination, influencing its chemical behavior in coordination chemistry.[18] This electrochemical nobility is quantified by its standard reduction potential for the half-reaction: \mathrm{Pt^{2+} + 2e^- \rightarrow Pt}, \quad E^\circ = +1.188 \, \mathrm{V} indicating a strong tendency to remain in the metallic state.[19] Platinum's catalytic activity arises from its ability to adsorb gases such as hydrogen and oxygen onto its surface without forming permanent chemical bonds, enabling efficient surface-mediated reactions in processes like hydrogenation and oxidation.[4] It is insoluble in water and most solvents but dissolves in fused alkalis under oxidizing conditions or in aqua regia, as noted earlier.[16] Regarding tarnish resistance, platinum shows no reaction with halogens below 300°C, contributing to its durability in ambient environments, though it forms intermetallic compounds with certain metals like cadmium and tin, altering its properties in alloys.[17][20]Isotopes
Platinum has 40 known isotopes, with mass numbers ranging from 165 to 206.[21] Of these, six are stable and occur naturally, while the remainder are radioactive. The stable isotopes are ¹⁹⁰Pt, ¹⁹²Pt, ¹⁹⁴Pt, ¹⁹⁵Pt, ¹⁹⁶Pt, and ¹⁹⁸Pt, with the following natural abundances:| Isotope | Natural Abundance (%) |
|---|---|
| ¹⁹⁰Pt | 0.014 |
| ¹⁹²Pt | 0.782 |
| ¹⁹⁴Pt | 32.967 |
| ¹⁹⁵Pt | 33.832 |
| ¹⁹⁶Pt | 25.242 |
| ¹⁹⁸Pt | 7.163 |