Osmium (Os)

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

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.