Figure 1. A bead of high-purity arc-melted osmium. Image copyright: smart-elements.com.

Osmium is the densest element.

Figure 2. Osmium is the densest element. A ball of it the size of a cricket ball would weigh 4.4 kilograms – the same as a women's shot put.

Osmium (Os) is a hard, silver-gray element in the platinum group and is as rare as gold (Figure 1). It is a transition element and is the densest of all elements – about twice as dense as lead (Figure 2). It was discovered by Smithson Tennant in London in 1803.


The chief source of osmium is as a by-product of smelting nickel. It is slowly oxidized in air. The most common isotope is 192Os (41.0%).


Like iridium, osmium is used in producing hard alloys. It is also used to make electrical contacts and pen points.


Osmium tetroxide is a colorless or faintly yellowish compound used to stain fats or as a fixative in the preparation of tissues for study by microscope. Osmium tetroxide evaporates readily, the vapor having a toxic action on the eyes, skin, and respiratory tract.


atomic number 76
relative atomic mass 190.2
relative density 22.57
melting point 3.045°C (5,513°F)
boiling point 5,027°C (9,081°F)