Osmium

Osmium is a Nobel metal and belongs to the platinum family. It is the densest and corrosion-resistant element. It was discovered in 1803.

History and Discovery

Osmium has an intertwined history with platinum and dates to the late 17th century. Initially, in 1748, platinum was identified as a distinct element and osmium was obtained as an insoluble impurity when platinum was dissolved in aqua regia. Later, in 1803, Smithson Tennant analyzed the insoluble fraction and identified two new elements, iridium and osmium. Osmium was identified due to its unique smell and formation of yellow solution when reacted with sodium hydroxide at high temperature. Osmium was name after the Greek work osme, which refers to smoky and shy smell due to the formation of osmium tetraoxide [1].

Osmium

Periodic Table ClassificationGroup 8
Period 6
State at 20CSolid
ColorSilvery, blue cast
Electron Configuration[Xe] 4f14 5d6 6s2
Electron Number76
Proton Number76
Electron Shell2, 8, 18, 32, 14, 2
Density22.60 g.cm-3 at 20°C
Atomic number76
Atomic Mass190.23 g.mol -1
Electronegativity according to Pauling2.20

Occurrence

Osmium is the one of the rarest natural element [2]. It is present in about 50 parts per trillion in the Earth’s crust. Osmium is present in nature in elemental form or as alloy with iridium. There are two most common alloys of osmium, iridosmium and osmiridium. Osmium is also found in alloys or copper and nickel, but they are quite rare. Currently, osmium is primarily extracted from nickel and platinum ores [3]. The largest natural reserves of osmium have been discovered in South Africa and are also present in Russia.

Physical Characteristics

Osmium is a silver metal with a grayish blue tint. It is very hard and brittle. Osmium belongs to the platinum family. It is considered as the densest metal and have a density of around 22.59 g/cm3 [4]. Osmium retains its lustrous shine even at high temperature and is very resistant to corrosion. Osmium has one of the highest melting and boiling points. It has the lowest vapor pressure among the members of platinum group.

Chemical Characteristics

There are various oxidation states of osmium, ranging from -2 to +8, and the most common are +2, +3 and +8. Osmium tetra-oxide exhibit the oxidation of +8 and is formed by the exposure of powdered osmium to air. This pale-yellow compound has a characteristic smell and is highly toxic. Osmium reacts with base, such as ammonia and forms nitride-osmates. Osmium reacts with halogens, and reacts with fluorine to form osmium pentafluoride, while forms tribromide, triiodide, and diiodide with other halogens.

Significance and Uses

  • Osmium alloys are used in making hard and high-wear items, including instrument pivots, tips of fountain pen, compasses and electrical contacts.
  • Osmium tetroxide is used for the detection of fingerprints.
  • Osmium tetroxide is used in electron microscopy for staining of fatty tissues. It is used in transmission electron microscopy (TEM) for biological studies.
  • Osmium is used in arthritis therapy through a technique termed as synovectomy.

Health Hazards

Osmium in elemental form is non-toxic, however, certain salts and compounds of osmium pose various health hazards. Osmium is powder form undergoes spontaneous inflammation when exposed to air, so care should be taken while its handling. Fumes of osmium tetroxide are very toxic and cause problems if inhaled, ingested or comes in contact with skin. It rapidly absorbs into the tissues and lead to severe irritation and lung congestion [5].

Isotopes of Osmium

Osmium has seven naturally occurring isotopes. There are six stable isotopes of osmium, osmium-184, osmium-187, osmium-188, osmium-189, osmium-190 and osmium-192. The most abundant stable isotope is osmium-192. The radioactive isotope of osmium is osmium-186 and has a very long half-life (2 × 1015 years).

REFERENCES

[1]. Weeks, M. E. (1968). Discovery of the Elements (7 ed.). Journal of Chemical Education. pp. 414–418. ISBN 978-0-8486-8579-9. OCLC 23991202.

[2]. “Reading: Abundance of Elements in Earth’s Crust | Geology”. courses.lumenlearning.com. Retrieved 2018-05-10

[3]. George, Micheal W. “2006 Minerals Yearbook: Platinum-Group Metals” (PDF). United States Geological Survey USGS. Retrieved 2008-09-16

[4]. Arblaster, J. W. (1995). “Osmium, the Densest Metal Known”. Platinum Metals Review. 39 (4): 164

[5]. George, Micheal W. “2006 Minerals Yearbook: Platinum-Group Metals” (PDF). United States Geological Survey USGS. Retrieved 2008-09-16