Osmium (Os)

Osmium is a chemical element with the atomic number 76 in the periodic table. It’s one of the rarest chemical elements that occur in Earth’s crust, with 1.5×10-3 ppm of natural abundance. 

Being a member of the platinum group metals (PGM), this noble metal with a pungent odor and high toxicity has many applications in medicine and various industries. As a part of the platinum metals group, osmium is often alloyed with the other precious metals. In this way, osmium alloys are used for an added durability, firmness, and anti-corrosive protection of manufactured items. 

Chemical and Physical Properties of Osmium

Property	Value
Symbol	Os
Name	Osmium
Atomic number	76
Atomic weight	190.23 g.mol-1
Group	Transition Metal
Period	6 (d-block)
Color	A bluish-white color with a silvery luster
Physical state	Solid at room temperature
Half-life	From 0.64(6) milliseconds to 2.0(11)×1015 years
Electronegativity	2.2
Density	22.40 g.cm-3 at 20°C
Melting point	3033°C, 5491°F, 3306 K
Boiling point	5008°C, 9046°F, 5281 K
Ionic radius	0.067 nm (+4)
Isotopes	37
Electronic shell	[Xe] 4f14 5d6 6s2
Uses	Used to tip gold pen points, instrument pivots, electric light filaments, high temp. alloys, pressure bearings. Very hard and corrosion-resistant.
Description	Heavy, brittle, white metal.
Shells	2,8,18,32,14,2
Valence	0,3,4,6,8
Crystal Structure	Hexagonal
Covalent Radius	1.26 Å
Atomic Radius	1.92 Å
Atomic Volume	08.49 cm³/mol
Name Origin	Greek: osmê (odor).
Discoverer	Smithson Tenant
Year	1804
Location	England
Pronunciation	OZ-mi-em
Oxydation States	2,3,(4),6,8

With the periodic table symbol Os, atomic number 76, atomic mass of 190.2 g.mol-1, and electron configuration [Xe] 4f14 5d- 6s2, osmium is a hard and brittle bluish-white metal with a silvery lustre and pungent odor.

Osmium reaches its boiling point at 5008°C, 9046°F, 5281 K, while the melting point is achieved at 3033°C, 5491°F, 3306 K. In this regard, this chemical element has one of the highest melting and boiling points. 

This member of the platinum metals family of elements has an electronegativity of 2.2 according to Pauling, whereas the atomic radius according to van der Waals is 0.136 nm. Osmium metal has an excellent anti-corrosive property, and is resistant to wear and tear. Even after exposure to high temperatures, osmium does not lose its silvery luster.

How Was Osmium Discovered?

Osmium was discovered by the English chemists Smithson Tennant and William Hyde Wollaston in 1803. At that time, some scientists believed that crude platinum powder was actually graphite (C), archaically known as plumbago. However, Tennant’s and Wollaston’s curiosity spurred them to conduct additional experiments to obtain more valid evidence on this claim. 

These two discoverers then decided to implement the method for the isolation of palladium and rhodium out of platinum ore that had been conceptualized by Wollaston. Namely, Tennant and Wollaston tried to dissolve a sample of crude platinum in aqua regia (a mixture of hydrochloric acid and nitric acid) in a London laboratory. 

This experiment resulted in a metallic black powder. But, the English chemists were still convinced that there’s something more to this graphite-resembling substance. So, they attempted mixing the powder with sodium hydroxide after which they heated the mixture. By adding water to the residue, the alkali was removed and mixed with hydrochloric acid. 

This step of the experiment proved to be revolutionary because Tennant and Wollaston discovered an unknown metal element exactly in this solution. From the new element referred to as ‘ptene’, they succeeded in isolating a substance with an extremely strong smell – osmium tetroxide (OsO4). 

How Did Osmium Get Its Name?

As a result of its exceptionally strong and unpleasant smell, element 76 was given the name ‘osmium’. The term is derived from the Greek word  ‘osme‘, which means ‘smelly’.

Where Can You Find Osmium?

Osmium is the rarest and least abundant transition metal that naturally occurs in Earth’s crust. It’s mainly traced in alloys together with other platinum metals. Furthermore, element 76 is also traced in the mineral osmiridium (also known as iridosmine – an alloy of osmium and iridium). Osmiridium deposits are most often found in the gold-bearing conglomerates located in the Ural Mountains (Russia), Witwatersrand (South Africa), and California and Oregon (United States). 

This toxic chemical element can also be found in the minerals siserskite, aurosmiridium, as well as in native platinum ores. For commercial purposes, osmium is mostly obtained as a byproduct of nickel refining, i.e. from the processing of platinum and nickel ores.

Osmium in Everyday Life

As a member of the platinum metals group, the main application of this transition metal is in the making of very hard alloys that characterize with great resistance to wear. Osmium can be also applied in the following instances:

• In medicine, osmium is used for making surgical implants, such as pacemakers and replacement heart valves;
• Due to its toughness and resistance to wear, osmium is used in the manufacturing of fountain pen tips and phonograph needles, instrument pivots, electrical contacts, and compass needles;
• Osmium tetroxide is often used as a fingerprint detection chemical, as well as in microscopy as a stain for fatty tissue;
• Synovectomy is a technique in medicine used for treatment of arthritis in which osmium is applied;
• In transmission electron microscopy (TEM), osmium is used as an intensifier of the image contrast;
• Together with iridium and rhenium, the 188Os isotope is applied in the dating of both terrestrial and meteoric rocks. 

How Dangerous Is Osmium?

Osmium tetroxide (OsO4) is considered as a highly toxic compound that may cause severe health problems, such as lung decongestion, eye irritation and inflammation, as well as skin damage. 

Upon inhalation of osmium fumes or metal dust particles, the following symptoms of osmium toxicity are typically  experienced:

• Severe headaches;
• Loss of breath and difficulty breathing;
• Wheezing;
• Abdominal cramps;
• Lung oedema;
• Kidney problems;
• Gastrointestinal problems;
• Eye damage;
• Blurred vision;
• Loss of vision;
• Skin burn and discoloration;
• Shock or collapse;
• Death.

Environmental Effects of Osmium

Osmium tetroxide is the main source of element 76 in the environment. It forms due to the fact that some compounds of this toxic chemical element are converted to tetroxide upon their exposure to air. For this reason, this osmium compound is referred to as an airborne toxin. 

Isotopes of Osmium

Naturally occurring osmium (76Os) is made up of seven isotopes:

• 184Os (0.02% of abundance);
• 186Os (1.6% of abundance); 
• 187Os (2.0% of abundance);
• 188Os (13.2% of abundance); 
• 189Os (16.1% of abundance);
• 190Os (26.3% of abundance);
• 192Os (40.8% of abundance).

There are 37 observed isotopes of osmium, with atomic masses ranging from osmium-161 to osmium-197. Among them, 30 are synthetically produced radioactive isotopes. Osmium-186 is the longest living form of element 76, with a half-life of 2.0(11)×1015 years. 

Nuclide [n 1]	Z	N	Isotopic mass (Da) [n 2][n 3]	Half-life [n 4]	Decay mode [n 5]	Daughter isotope [n 6]	Spin and parity [n 7][n 8]	Natural abundance (mole fraction)
	Excitation energy							Normal proportion	Range of variation
161Os	76	85		0.64(6) ms	α	157W
162Os	76	86	161.98443(54)#	1.87(18) ms	α	158W	0+
163Os	76	87	162.98269(43)#	5.5(6) ms	α	159W	7/2−#
					β+, p (rare)	162W
					β+ (rare)	163Re
164Os	76	88	163.97804(22)	21(1) ms	α (98%)	160W	0+
					β+ (2%)	164Re
165Os	76	89	164.97676(22)#	71(3) ms	α (60%)	161W	(7/2−)
					β+ (40%)	165Re
166Os	76	90	165.972691(20)	216(9) ms	α (72%)	162W	0+
					β+ (28%)	166Re
167Os	76	91	166.97155(8)	810(60) ms	α (67%)	163W	3/2−#
					β+ (33%)	167Re
168Os	76	92	167.967804(13)	2.06(6) s	β+ (51%)	168Re	0+
					α (49%)	164W
169Os	76	93	168.967019(27)	3.40(9) s	β+ (89%)	169Re	3/2−#
					α (11%)	165W
170Os	76	94	169.963577(12)	7.46(23) s	β+ (91.4%)	170Re	0+
					α (8.6%)	166W
171Os	76	95	170.963185(20)	8.3(2) s	β+ (98.3%)	171Re	(5/2−)
					α (1.7%)	167W
172Os	76	96	171.960023(16)	19.2(5) s	β+ (98.9%)	172Re	0+
					α (1.1%)	168W
173Os	76	97	172.959808(16)	22.4(9) s	β+ (99.6%)	173Re	(5/2−)
					α (.4%)	169W
174Os	76	98	173.957062(12)	44(4) s	β+ (99.97%)	174Re	0+
					α (.024%)	170W
175Os	76	99	174.956946(15)	1.4(1) min	β+	175Re	(5/2−)
176Os	76	100	175.95481(3)	3.6(5) min	β+	176Re	0+
177Os	76	101	176.954965(17)	3.0(2) min	β+	177Re	1/2−
178Os	76	102	177.953251(18)	5.0(4) min	β+	178Re	0+
179Os	76	103	178.953816(19)	6.5(3) min	β+	179Re	(1/2−)
180Os	76	104	179.952379(22)	21.5(4) min	β+	180Re	0+
181Os	76	105	180.95324(3)	105(3) min	β+	181Re	1/2−
182Os	76	106	181.952110(23)	22.10(25) h	EC	182Re	0+
183Os	76	107	182.95313(5)	13.0(5) h	β+	183Re	9/2+
184Os	76	108	183.9524891(14)	Observationally Stable[n 9]			0+	2(1)×10−4
185Os	76	109	184.9540423(14)	93.6(5) d	EC	185Re	1/2−
186Os[n 10]	76	110	185.9538382(15)	2.0(11)×1015 y	α	182W	0+	0.0159(3)
187Os[n 11]	76	111	186.9557505(15)	Observationally Stable[n 12]			1/2−	0.0196(2)
188Os[n 11]	76	112	187.9558382(15)	Observationally Stable[n 13]			0+	0.1324(8)
189Os	76	113	188.9581475(16)	Observationally Stable[n 14]			3/2−	0.1615(5)
190Os	76	114	189.9584470(16)	Observationally Stable[n 15]			0+	0.2626(2)
191Os	76	115	190.9609297(16)	15.4(1) d	β−	191Ir	9/2−
192Os	76	116	191.9614807(27)	Observationally Stable[n 16]			0+	0.4078(19)
193Os	76	117	192.9641516(27)	30.11(1) h	β−	193Ir	3/2−
194Os	76	118	193.9651821(28)	6.0(2) y	β−	194Ir	0+
195Os	76	119	194.96813(54)	6.5 min	β−	195Ir	3/2−#
196Os	76	120	195.96964(4)	34.9(2) min	β−	196Ir	0+
197Os	76	121		2.8(6) min

Source: Wikipedia

List of Osmium Compounds 

When osmium participates in a compound, it can occur in the oxidation states of +2 to +8. However, this transition metal most commonly adopts the +2, +3, +4, and +8 oxidation states. 

Osmium forms unstable compounds with alkali elements, while it produces the highly toxic chlorine gas in reaction with hydrochloric acid. Osmium also reacts with base compounds, fluorine, and halogens. 

When osmium powder is exposed to air, osmium tetra-oxide is formed. Osmium(VIII) oxide (OsO4) is a powerful oxidizing agent and highly toxic osmium compound with a pale-yellow color. It carries the characteristic pungent smell associated with this chemical element. 

The list of most commonly prepared osmium compounds contains the following items:

• Osmocene
• Osmiridium
• Osmium borides
• Osmium dioxide
• Osmium hexafluoride
• Osmium pentacarbonyl
• Osmium pentafluoride
• Osmium tetroxide
• Osmium(IV) chloride
• Potassium osmate
• Triosmium dodecacarbonyl

5 Interesting Facts and Explanations

1. 1803 was a fruitful year for English scientists. While Smithson Tennant discovered the chemical elements osmium and iridium the same year, Wollaston also succeeded to discover two other chemicals – palladium and rhodium. 
2. While performing his chemical experiments and studies, Tennant also became the first scientist to determine that diamonds are pure carbon. 
3. Osmium is the densest naturally occurring stable chemical element on Earth. 
4. Out of all elements that comprise the PMG – platinum metals group (ruthenium, rhodium, palladium, iridium, and platinum), osmium has the highest melting point and the lowest vapor pressure. 
5. Element 76 has no known biological role.