---
title: "Iridium (Ir) — element 77"
description: "Iridium is element 77, symbol Ir, atomic weight 192.22. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/iridium/
site: "At One Place"
---

# Iridium (Ir)

**Iridium — element 77: Ir**

192.22 u · Transition metal · period 6, group 9

A thin layer of iridium in 66-million-year-old clay, found worldwide by Luis and Walter Alvarez in 1980, is the evidence that an asteroid killed the dinosaurs: iridium is vanishingly rare in the Earth's crust because it sank into the core, but is comparatively common in meteorites, so a global iridium spike means something very large arrived from space. It is the most corrosion-resistant metal known, unattacked by any acid including aqua regia. Until 2019 the kilogram was defined by a cylinder of 90% platinum and 10% iridium kept near Paris.

## Properties

- **Atomic number:** 77
- **Symbol:** Ir
- **Standard atomic weight:** 192.22 u
- **Category:** Transition metal
- **Group:** 9
- **Period:** 6
- **Block:** d-block
- **Electron configuration:** [Xe] 6s² 4f¹⁴ 5d⁷
- **Electrons per shell:** 2, 8, 18, 32, 15, 2
- **Melting point:** 2,719 K — 2445.8 °C — 4434.5 °F
- **Boiling point:** 4,701 K — 4427.9 °C — 8002.1 °F
- **Density:** 22.42 g/cm³
- **Electronegativity:** 2.2 (Pauling)
- **Oxidation states:** +4, +3
- **Discovered:** 1803 by Smithson Tennant

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁷
- **Noble gas shorthand:** [Xe] 6s² 4f¹⁴ 5d⁷
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 15 · P: 2

## What it is used for

- Long-life spark plug electrodes
- Crucibles for growing sapphire and other high-temperature crystals
- Iridium-192 sources for industrial radiography of welds
- Electrodes for producing chlorine and for hydrogen electrolysers
- Deep-space probe casings for radioisotope power sources

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 75 | Rhenium | Re | 186.21 |
| 76 | Osmium | Os | 190.23 |
| 77 | Iridium ← | Ir | 192.22 |
| 78 | Platinum | Pt | 195.08 |
| 79 | Gold | Au | 196.97 |

## Group 9

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Iridium sits in group 9 with cobalt, rhodium, meitnerium.

## Common questions

### What is the atomic number of iridium?

77. That is the number of protons in its nucleus, which is what defines the element.

### What is the symbol for iridium?

Ir.

### What is the electron configuration of iridium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁷, or [Xe] 6s² 4f¹⁴ 5d⁷ in noble-gas shorthand.

### What is the melting point of iridium?

2,719 K — 2445.8 °C — 4434.5 °F

## Related pages

- [Rhenium (Re)](https://atoneplace.net/element/rhenium/)
- [Osmium (Os)](https://atoneplace.net/element/osmium/)
- [Platinum (Pt)](https://atoneplace.net/element/platinum/)
- [Gold (Au)](https://atoneplace.net/element/gold/)
- [Cobalt (Co)](https://atoneplace.net/element/cobalt/)
- [Rhodium (Rh)](https://atoneplace.net/element/rhodium/)
- [Meitnerium (Mt)](https://atoneplace.net/element/meitnerium/)

## Sources

- Official vendor product documentation — At One Place (/about/)