Osmium vs Iridium
Both are transition metals, so they share the same broad chemistry.
The differences in plain English
- They share period 6, the same row, so their outer electrons are in the same shell but they differ in how many there are: Osmium has 2, Iridium has 2.
- Both are solids at room temperature.
- Osmium is denser, only slightly more than Iridium (22.59 against 22.56 g/cm³).
- Osmium melts at 3033 °C, far above Iridium at 2446 °C.
- An atom of Iridium is only slightly heavier than an atom of Osmium (192.22 u against 190.23 u).
- Iridium pulls electrons more strongly in a bond (electronegativity 2.2 against 2.2).
Side by side
| Osmium | Iridium | |
|---|---|---|
| Atomic number | 76 | 77 |
| Symbol | Os | Ir |
| Family | Transition metal | Transition metal |
| Group / period | 8 / 6 | 9 / 6 |
| State at room temperature | Solid | Solid |
| Atomic mass | 190.233 u | 192.217 u |
| Density | 22.59 g/cm³ | 22.56 g/cm³ |
| Melting point | 3033 °C | 2446 °C |
| Boiling point | 5012 °C | 4130 °C |
| Electronegativity | 2.2 | 2.2 |
| First ionisation energy | 840 kJ/mol | 880 kJ/mol |
| Electron configuration | [Xe] 4f14 5d6 6s2 | [Xe] 4f14 5d7 6s2 |
| Discovered by | Smithson Tennant | Smithson Tennant |
Osmium
Osmium is a hard, brittle metal with a faint blue sheen, and it holds a record that no other natural element can beat: it is the densest of them all. A piece of osmium weighs almost three times as much as the same sized piece of iron and about twice as much as a lump of lead. It belongs to the platinum group, a cluster of six rare, unreactive metals that sit together in the middle of the periodic table.
- Osmium and iridium are so close in density, 22.59 and 22.56 grams per cubic centimetre, that for much of the twentieth century scientists disagreed about which was heavier.
- The lighting company Osram takes its name from osmium and wolfram, the German word for tungsten, the two metals used in early light bulb filaments.
Iridium
Ask a geologist about iridium and they will tell you about a layer of clay. Found in rocks all over the world and dated to about 66 million years ago, it contains far more iridium than normal crustal rock. Its discovery in 1980 gave the first hard evidence that a giant asteroid struck the Earth at the moment the dinosaurs vanished.
- The worldwide iridium-rich clay layer, identified by Luis and Walter Alvarez and colleagues in 1980, is key evidence for the asteroid impact linked to the extinction of the dinosaurs.
- A cylinder of platinum-iridium alloy kept near Paris defined the kilogram from 1889 until 2019.
Colour the bubbles by density or melting point and the difference jumps out.
More comparisons
Data from Periodic-Table-JSON (CC BY-SA); temperatures rounded to the nearest degree. Written by Anthony.