Osmium (from Greek osme (ὀσμή) meaning "smell") is a chemical element with symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest known element, with a density of 22.59 g/cm3. Its alloys with platinum, iridium, and other platinum group metals are employed in fountain pen nibs, electrical contacts, and other applications where extreme durability and hardness are needed.
Osmium reacts with oxygen to produce osmium tetroxide, OsO4, This compound, which boils at 130 oC and has a strong, acrid odor, is the source of osmium's name. At ambient temperature, this reaction is slow, so pieces of osmium metal appear to be corrosion-resistant. When finely-divided, though osmium catches fire spontaneously. Since its principal oxide is OsO4, bulk Os metal is at risk of igniting if its temperature is above 130 oC.
NUCLEAR PROPERTIES AND FORMATION
At least 100 isotopes of Os have been predicted, ranging from 152Os to 252Os, of which 36 have been observed (plus 9 isomers). The heaviest of these, 193Os and above, decay by beta emission and are short-lived (with the exception of 194Os, which has a 4 yr half-life). At 183Os and below, isotopes decay by a mixture of positive beta decay (electron capture or positron emission) and alpha emission. Between 183Os and 175Os, positive beta decay is the only mode observed. Between 174Os and 168Os, beta decay is dominant, but an alpha decay branch is also active. Between 167Os and 163Os, alpha decay dominates over positive beta decay. Alpha decay is observed or predicted to be the dominant (possibly only) active decay mode down to 160Os. Note that neutron count of 160Os is 84, two above the shell closure at N = 82. Positive beta decay and proton decay are predicted below 160Os.
Osmium has a stable band ranging from 192Os down to 184Os. Of these, 192Os and the band 190Os to 187Os are both effectively and observationally stable, while 186Os and 184Os are effectively stable but known to emit alpha radiation. The remaining two isotopes in this band 191Os and 185Os are short-lived and beta-decaying (positive at 185Os).
Isotopes 188Os and heavier are the endpoints of beta decay chains originating with neutron-rich nuclei produced during a core-collapse supernova or a neutron star merger. These are the most common isotopes, with 192Os accounting for 0.408 of all atoms, 190Os accounting for 0.263, 189Os accounting for 0.161, and 188Os accounting for 0.132. It is possible for 186Os to form via neutron capture by 185Re, followed by beta decay. An additional neutron capture would give 187Os. Alternately, gamma capture followed by neutron emission can produce 187Os from 188Os and 186Os from 187Os. If the environment is sufficiently hot that gammas with enough energy are common, gamma capture & neutron emission can produce 185Os; and, since it has a 93 day half-life, some 184Os can be produced from 185Os.
Aside from the stable ones, the only isotope of osmium which persists (in theory) for more than 25 yrs after a formation event is 194Os, which has a 6.0 yr half-life.