Osmium - Os, 76

Osmium has a melting point of 3045 +/- 30 °C, boiling point of 5027 +/- 100°C, specific gravity of 22.57, with a valence usually +3, +4, +6, or +8, but sometimes 0, +1, +2, +5, +7. It is a lustrous blue-white metal. It is very hard and remains brittle even at high temperatures. Osmium has the lowest vapor pressure and highest melting point of the platinum group metals. Although solid osmium is unaffected by air at room temperature, the powder will give off osmium tetroxide, a strong oxidizer, highly toxic, with a characteristic odor (hence the metal's name). Osmium is slightly more dense than iridium, so osmium is often credited as being the heaviest element (calculated density ~ 22.61). The calculated density for iridium, based on its space lattice, is 22.65, though the element hasn't been measured as heavier than osmium.

Osmium tetroxide can be used to stain fatty tissue for microscope slides and to detect fingerprints. Osmium is used to add hardness to alloys. It is also used for fountain pen tips, instrument pivots, and electrical contacts.