Iridium
Iridium is a hard, brittle, silvery-white transition metal and one of the two densest elements known, alongside its close neighbor osmium. It's prized above all for its extreme resistance to corrosion and heat — it doesn't react with acids, oxygen, or almost anything else even at very high temperatures, which makes it one of the most chemically stable metals on the periodic table.
- Group · Period
- 9 · 6
- At room temp
- solid
- Melts at
- 2719 K
- Density
- 22.42 g/cm³
- Discovered
- 1803
Uses
Iridium’s defining industrial use is as a hardening agent and corrosion barrier: alloyed with platinum, it’s used to make crucibles that can hold molten materials — including molten oxides used to grow synthetic crystals for lasers and electronics — at temperatures that would destroy almost any other container. Iridium-tipped electrodes and spark plugs last far longer than conventional ones because the metal barely wears down, and the same toughness makes it useful in deep-sea cable components and specialized scientific instruments that must survive harsh, corrosive environments for decades.
Historically, an alloy of platinum and iridium was chosen for the international prototype kilogram and meter bar, the physical objects that once defined those units for the world, precisely because the alloy resists tarnishing and dimensional change over long timescales. Iridium-192 is also used as a radiation source in industrial radiography, for inspecting welds and castings, and in a cancer treatment technique called brachytherapy.
History
Iridium was discovered in England in 1803 by chemist Smithson Tennant, who was investigating the dark, insoluble residue left behind after dissolving crude platinum ore in acid. Within that residue he identified two new metals: osmium and iridium. He named iridium after Iris, the Greek goddess of the rainbow, because the salts it forms come in such a striking range of colors. Because it’s so hard and has such a high melting point, iridium metal proved difficult to work with, and it took decades of refinement in metallurgy before it could be reliably purified and shaped for practical use.
Fun facts
- Iridium and osmium are essentially tied for the title of densest naturally occurring element — both pack roughly twice the density of lead into the same volume.
- A thin, unusually iridium-rich layer of clay found worldwide at the boundary between the Cretaceous and Paleogene periods is key evidence linking the extinction of the dinosaurs to a massive asteroid impact.
- Iridium is so rare in Earth's crust that essentially none of it is mined directly — it's recovered as a byproduct of nickel and platinum ore processing.
Frequently asked questions
Why is iridium linked to the extinction of the dinosaurs?
Asteroids and comets contain far more iridium than Earth's crust does, since most of Earth's original iridium sank into the planet's core when it was still molten. Geologist Walter Alvarez and his father, physicist Luis Alvarez, found a spike of iridium in rock layers from exactly 66 million years ago worldwide, which strongly supports the idea that a large asteroid struck Earth around that time.
What makes iridium so resistant to corrosion?
Its electron structure makes iridium extremely unreactive — acids, including the notoriously aggressive aqua regia that dissolves gold, barely touch it at room temperature. This same stability is why iridium-tipped spark plugs and lab crucibles can survive years of extreme heat and chemical exposure without wearing out.
Is iridium more valuable than gold?
Iridium is generally rarer and, at least at some points, has traded for more per ounce than gold, though its price swings a lot more because the market is small and tightly tied to industrial demand rather than jewelry or investment demand.
Compounds
3 notable compounds containing Ir
- IrF6halide
Iridium hexafluoride
A volatile, deep yellow solid and one of the most powerful oxidizing agents among the transition-metal fluorides, decomposing readily and attacking glassware.
Used for: Research reagent for fluorination chemistry
- IrCl3salt
Iridium(III) chloride
A dark green to black crystalline solid with iridium in its +3 oxidation state. The anhydrous form is chemically inert and barely soluble, while the hydrated form dissolves readily in water.
Used for: Starting material for synthesizing other iridium catalysts and complexes
- IrO2oxide
Iridium(IV) oxide
A blue-black oxide with a rutile-type crystal structure that, unusually for a simple metal oxide, conducts electricity almost like a metal.
Used for: Electrode coatings for chlorine production and water electrolysis
Isotopes
42 known isotopes
Swipe to see all columns →
| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Ir-191 stable | 37.3% | Stable | — | -36,708.83 keV | 7,948.11 keV | — |
| Ir-193 stable | 62.7% | Stable | — | -34,536.31 keV | 7,938.13 keV | — |
Show all 42 isotopes
| Ir-164 | — | — | Electron capture / beta-plus decay, Alpha decay, Proton emission | -7,483 keV | 7,750 keV | — |
| Ir-165 | — | 1 us | Proton emission, Alpha decay | -11,595 keV | 7,776 keV | 4.56 × 10-10 eV |
| Ir-166 | — | 10.5 ms | Alpha decay (93%), Proton emission (7%) | -13,306 keV | 7,789 keV | 4.35 × 10-14 eV |
| Ir-167 | — | 35.2 ms | Alpha decay (48%), Proton emission (32%), Electron capture / beta-plus decay (20%) | -17,072.45 keV | 7,812.83 keV | 1.30 × 10-14 eV |
| Ir-168 | — | 222 ms | Alpha decay (100%), Electron capture / beta-plus decay, Proton emission | -18,666.23 keV | 7,823.85 keV | 2.06 × 10-15 eV |
| Ir-169 | — | 0.353 s | Alpha decay (45%), Electron capture / beta-plus decay, Proton emission | -22,093.42 keV | 7,845.59 keV | 1.29 × 10-15 eV |
| Ir-170 | — | 0.87 s | Electron capture / beta-plus decay (94.8%), Alpha decay (5.2%) | -23,182 keV | 7,853 keV | 5.24 × 10-16 eV |
| Ir-171 | — | 3.2 s | Electron capture / beta-plus decay (85%), Alpha decay (15%) | -26,412.03 keV | 7,873.49 keV | 1.43 × 10-16 eV |
| Ir-172 | — | 4.4 s | Electron capture / beta-plus decay (98%), Alpha decay (2%) | -27,379.37 keV | 7,880.26 keV | 1.04 × 10-16 eV |
| Ir-173 | — | 9.0 s | Electron capture / beta-plus decay (93%), Alpha decay (7%) | -30,268.46 keV | 7,898.07 keV | 5.07 × 10-17 eV |
| Ir-174 | — | 7.9 s | Electron capture (99.5%), Alpha decay (0.5%) | -30,785.94 keV | 7,902.04 keV | 5.78 × 10-17 eV |
| Ir-175 | — | 9 s | Electron capture / beta-plus decay (99.15%), Alpha decay (0.85%) | -33,394.51 keV | 7,917.91 keV | 5.07 × 10-17 eV |
| Ir-176 | — | 8.7 s | Electron capture / beta-plus decay (96.9%), Alpha decay (3.1%) | -33,881.92 keV | 7,921.55 keV | 5.24 × 10-17 eV |
| Ir-177 | — | 29.8 s | Electron capture / beta-plus decay (99.94%), Alpha decay (0.06%) | -36,047.43 keV | 7,934.63 keV | 1.53 × 10-17 eV |
| Ir-178 | — | 12 s | Electron capture / beta-plus decay (100%) | -36,254.31 keV | 7,936.56 keV | 3.80 × 10-17 eV |
| Ir-179 | — | 79 s | Electron capture / beta-plus decay (100%) | -38,081.72 keV | 7,947.52 keV | 5.78 × 10-18 eV |
| Ir-180 | — | 1.5 m | Electron capture / beta-plus decay (100%) | -37,977.53 keV | 7,947.63 keV | 5.07 × 10-18 eV |
| Ir-181 | — | 4.90 m | Electron capture / beta-plus decay (100%) | -39,463.04 keV | 7,956.52 keV | 1.55 × 10-18 eV |
| Ir-182 | — | 15.0 m | Electron capture / beta-plus decay (100%) | -39,051.68 keV | 7,954.89 keV | 5.07 × 10-19 eV |
| Ir-183 | — | 58 m | Electron capture / beta-plus decay (100%) | -40,202.14 keV | 7,961.82 keV | 1.31 × 10-19 eV |
| Ir-184 | — | 3.09 h | Electron capture / beta-plus decay (100%) | -39,610.86 keV | 7,959.2 keV | 4.10 × 10-20 eV |
| Ir-185 | — | 14.4 h | Electron capture / beta-plus decay (100%) | -40,335.56 keV | 7,963.72 keV | 8.80 × 10-21 eV |
| Ir-186 | — | 16.64 h | Electron capture / beta-plus decay (100%) | -39,172.35 keV | 7,958.05 keV | 7.62 × 10-21 eV |
| Ir-187 | — | 10.5 h | Electron capture / beta-plus decay (100%) | -39,549.38 keV | 7,960.67 keV | 1.21 × 10-20 eV |
| Ir-188 | — | 41.5 h | Electron capture / beta-plus decay (100%) | -38,344.96 keV | 7,954.85 keV | 3.05 × 10-21 eV |
| Ir-189 | — | 13.2 d | Electron capture (100%) | -38,449.65 keV | 7,956.02 keV | 4.00 × 10-22 eV |
| Ir-190 | — | 11.78 d | Electron capture / beta-plus decay (100%), Beta-plus decay (0.002%) | -36,753.61 keV | 7,947.7 keV | 4.48 × 10-22 eV |
| Ir-192 | — | 73.829 d | Beta-minus decay (95.24%), Electron capture (4.76%) | -34,835.63 keV | 7,939 keV | 7.15 × 10-23 eV |
| Ir-194 | — | 19.18 h | Beta-minus decay (100%) | -32,531.78 keV | 7,928.49 keV | 6.61 × 10-21 eV |
| Ir-195 | — | 2.29 h | Beta-minus decay (100%) | -31,692.32 keV | 7,924.92 keV | 5.53 × 10-20 eV |
| Ir-196 | — | 52 s | Beta-minus decay (100%) | -29,435.52 keV | 7,914.15 keV | 8.77 × 10-18 eV |
| Ir-197 | — | 5.8 m | Beta-minus decay (100%) | -28,264.12 keV | 7,909 keV | 1.31 × 10-18 eV |
| Ir-198 | — | 8 s | Beta-minus decay (100%) | -25,710 keV | 7,897 keV | 5.70 × 10-17 eV |
| Ir-199 | — | 6 s | Beta-minus decay | -24,398.53 keV | 7,891.21 keV | 7.60 × 10-17 eV |
| Ir-200 | — | 300 ns | Beta-minus decay | -21,570 keV | 7,878 keV | 1.52 × 10-9 eV |
| Ir-201 | — | 21 s | Beta-minus decay (100%) | -19,840 keV | 7,870 keV | 2.17 × 10-17 eV |
| Ir-202 | — | 11 s | Beta-minus decay (100%) | -16,640 keV | 7,855 keV | 4.15 × 10-17 eV |
| Ir-203 | — | — | Beta-minus decay (100%) | -14,370 keV | 7,845 keV | — |
| Ir-204 | — | 160 ns | Beta-minus decay, Beta-minus, neutron emission | -9,570 keV | 7,823 keV | 2.85 × 10-9 eV |
| Ir-205 | — | — | Beta-minus decay (100%), Beta-minus, neutron emission | -5,600 keV | 7,805 keV | — |