Atomic number 77

Iridium

Ir
+ phase
− phase

Shells Orbitals 4f 5d 6s

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77 electrons in 6 shells: 2, 8, 18, 32, 15, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 6
Group: 9
Block: d
Standard atomic weight: 192.22(1) Da
Electron configuration: [Xe] 4f¹⁴ 5d⁷ 6s²
Phase at 25 °C: solid
Melting point: 2446 °C
Boiling point: 4130 °C
Density: 22.42 g/cm³
Electronegativity (Pauling): 2.2
Discovered: 1803, Smithson Tennant

Neighbours in the table

Os Pt

Salts of iridium come in so many colours that the element was named for Iris, the Greek goddess of the rainbow, according to IUPAC’s atomic-weights commission (CIAAW). Smithson Tennant found the metal in 1803, Jefferson Lab records, in black residue left after platinum ore was dissolved in aqua regia, a mixture of nitric and hydrochloric acids. Osmium came out of the same residue.

Iridium’s shells run 2, 8, 18, 32, 15 and 2, for 77 electrons, written as [Xe] 4f¹⁴ 5d⁷ 6s². In OpenStax’s account, two series start at lanthanum. Up to lutetium, fourteen f electrons take the fourth shell from 18 to 32. Up to gold, ten d electrons fill the shell below the outermost; iridium has seven. Jefferson Lab lists +3 and +4 as its common oxidation states, the charges it would carry if its bonds were counted as fully ionic. Los Alamos lists a span from −3 to +9, and OpenStax dates the +9 state to 2014.

Los Alamos describes the metal as white, like platinum, with a faint yellowish tint. It is hard and brittle, which makes it difficult to machine or shape. OpenStax ties the high melting points of transition metals to strong metallic bonding, and iridium melts at 2446 °C.

Where it is found

In nature iridium occurs as the uncombined metal, mixed with platinum and its relatives in alluvial deposits, the sands and gravels laid down by rivers, Los Alamos says. The crust holds about 1 × 10⁻³ milligrams per kilogram, in Jefferson Lab’s figure. Jefferson Lab names two present-day sources, platinum ores and nickel mining.

A thin iridium-rich layer of sediment lies across the whole planet at the end of the Cretaceous period, Jefferson Lab notes. Meteors and asteroids carry a higher share of iridium than Earth’s crust does, so this enriched band is seen as evidence of an impact. In Jefferson Lab’s account, dust from the collision would have spread the iridium worldwide and blocked the sun for a time. Many plant and animal species, the dinosaurs among them, would have died out as a result.

South Africa leads the world in mined platinum-group material, and its Bushveld Complex holds the largest reserves of these metals, the US Geological Survey reports. For iridium alone the Survey gives American imports: an estimated 1,400 kilograms in 2024, down from 2,040 in 2023.

Why it matters

The standard metre bar kept in Paris was an alloy of 90 per cent platinum and 10 per cent iridium, Los Alamos records. Jefferson Lab gives the reason for the choice: in its words iridium resists corrosion better than any other metal known. The bar defined the metre until 1960. The unit was then redefined using an orange-red line in the spectrum of krypton-86.

Iridium’s main job, according to Los Alamos, is hardening platinum. It also goes into crucibles and other equipment that must survive high temperatures, and into electrical contacts. Alloyed with osmium, it tips fountain pens and compass bearings. A thin coating on certain optical lenses cuts glare and sharpens the image in some lighting, Los Alamos adds.

Iridium-192 serves as a radiation source in gamma cameras that check aircraft parts, boilers and pipeline welds for manufacturing flaws without harming them, IUPAC notes. Hospitals use the same isotope in brachytherapy, radiation treatment given from a source placed inside the body. A far shorter-lived form, iridium-191m, with a half-life of 5 seconds, images blood flow, especially in children.

The Survey put iridium’s average 2024 price at about 4,800 dollars a troy ounce, 3 per cent above 2023. Rhodium, palladium and ruthenium all fell over the same year, and platinum slipped slightly.

Properties

Properties of iridium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	77
Standard atomic weight	192.22(1) Da	Da	IUPAC/CIAAW 2024
Melting point	2446 °C	°C	Los Alamos: 2446 °C
Boiling point	4130 °C	°C	Los Alamos: 4130 °C
Density	22.42 g/cm³	g/cm³
Electronegativity (Pauling)	2.2
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁷
Phase at 25 °C	solid

Safety

A solid piece of iridium poses little risk: Haz-Map, quoted in PubChem, finds the platinum-group metals generally nontoxic as metals. The concern is fine powder, which the Hazardous Substances Data Bank says may ignite on its own in air. Of 249 company reports, 36.9% find no hazard; among those naming one, 18.1% class iridium as a flammable solid, which gives its record the signal word “Danger”. The PubChem Laboratory Chemical Safety Summary for iridium holds the detail, including reports of serious eye irritation and of possible long-lasting harm to aquatic life.

Questions

Is iridium denser than osmium?

Only in calculation. Los Alamos reports that calculations from the crystal lattice of each metal give 22.65 g/cm³ for iridium and 22.61 for osmium, a narrow lead for iridium. Direct measurements tip the other way, putting osmium slightly ahead. PubChem's record lists iridium at 22.42 g/cm³, so a litre of it would weigh over 22 kilograms.

Can acid dissolve iridium?

No acid attacks the metal, Los Alamos reports, and that includes aqua regia, the nitric and hydrochloric acid mixture that dissolves crude platinum. Molten salts are a different case: fused sodium chloride and sodium cyanide do attack it. Its ions are less robust. OpenStax notes that Ir²⁺ is unstable and reacts readily with oxygen in air, and that the platinum metals form simple cations stable in water only with difficulty.

Who discovered iridium?

The British chemist Smithson Tennant is credited with it, in 1803; Jefferson Lab says he found osmium at the same time. CIAAW's history in PubChem's record adds a second discoverer, the French chemist H. V. Collet-Descotils, who worked independently and published a month earlier. Tennant kept the credit, CIAAW suggests, perhaps because he was the only one to find osmium as well.

Is iridium radioactive?

Natural iridium is not. It is a mixture of two stable isotopes, iridium-191 at about 37 per cent and iridium-193 at about 63 per cent, in CIAAW's abundances. Radioactive iridium-192 is produced from those stable isotopes. Its half-life is 73.82 days in the nuclear data of PubChem's record. Industry uses it as the radiation source in gamma cameras, and hospitals use it in brachytherapy, treatment from a source placed inside the body.