---
title: "Osmium (Os) — element 76"
description: "Osmium is element 76, symbol Os, atomic weight 190.23. Configuration, properties, uses and neighbours."
url: https://atoneplace.net/element/osmium/
site: "At One Place"
---

# Osmium (Os)

**Osmium — element 76: Os**

190.23 u · Transition metal · period 6, group 8

Osmium is the densest naturally occurring element — a litre of it weighs about 22.6 kg, roughly twice a litre of lead — though its margin over iridium is only about a tenth of a percent and which of the two wins depends on whose measurement you take. Its tetroxide is the reason for the name, from the Greek for smell: it is volatile, reeks of chlorine at parts per billion, and will blind you by fixing the cornea, which is a hazard almost unique among laboratory reagents. Fixing biological tissue for electron microscopy still relies on it, because it binds to fats and is opaque to electrons.

## Properties

- **Atomic number:** 76
- **Symbol:** Os
- **Standard atomic weight:** 190.23 u
- **Category:** Transition metal
- **Group:** 8
- **Period:** 6
- **Block:** d-block
- **Electron configuration:** [Xe] 6s² 4f¹⁴ 5d⁶
- **Electrons per shell:** 2, 8, 18, 32, 14, 2
- **Melting point:** 3,306 K — 3032.8 °C — 5491.1 °F
- **Boiling point:** 5,285 K — 5011.9 °C — 9053.3 °F
- **Density:** 22.57 g/cm³
- **Electronegativity:** 2.2 (Pauling)
- **Oxidation states:** +4, +3, +8
- **Discovered:** 1803 by Smithson Tennant

## Electron configuration

- **Full:** 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁶
- **Noble gas shorthand:** [Xe] 6s² 4f¹⁴ 5d⁶
- **Shells:** K: 2 · L: 8 · M: 18 · N: 32 · O: 14 · P: 2

## What it is used for

- Fountain pen nib tips and phonograph styli, alloyed with iridium
- Osmium tetroxide as a stain for fat in electron microscopy
- Osmium tetroxide as a catalyst in organic synthesis
- Electrical contacts that must not wear
- Osmium-rhenium dating of ore deposits and meteorite impacts

## Hazards

- Osmium tetroxide is volatile, extremely toxic and damages the eyes at very low concentrations
- Osmium powder oxidises in air to the tetroxide at room temperature

> Osmium and iridium are within a fraction of a percent of each other and sources disagree about which is denser. Values calculated from X-ray lattice parameters give osmium 22.587 and iridium 22.562 g/cm³; the directly measured values quoted here, 22.57 and 22.42, give osmium the same edge by a wider margin.

## Neighbours in the table

| Z | Element | Symbol | Mass |
| --- | --- | --- | --- |
| 74 | Tungsten | W | 183.84 |
| 75 | Rhenium | Re | 186.21 |
| 76 | Osmium ← | Os | 190.23 |
| 77 | Iridium | Ir | 192.22 |
| 78 | Platinum | Pt | 195.08 |

## Group 8

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Osmium sits in group 8 with iron, ruthenium, hassium.

## Common questions

### What is the atomic number of osmium?

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

### What is the symbol for osmium?

Os.

### What is the electron configuration of osmium?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁶, or [Xe] 6s² 4f¹⁴ 5d⁶ in noble-gas shorthand.

### What is the melting point of osmium?

3,306 K — 3032.8 °C — 5491.1 °F

## Related pages

- [Tungsten (W)](https://atoneplace.net/element/tungsten/)
- [Rhenium (Re)](https://atoneplace.net/element/rhenium/)
- [Iridium (Ir)](https://atoneplace.net/element/iridium/)
- [Platinum (Pt)](https://atoneplace.net/element/platinum/)
- [Iron (Fe)](https://atoneplace.net/element/iron/)
- [Ruthenium (Ru)](https://atoneplace.net/element/ruthenium/)
- [Hassium (Hs)](https://atoneplace.net/element/hassium/)

## Sources

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