what does it react when it comes in contact with other elements?
Osmium tetroxide  is formed when powdered osmium is exposed to air,  it  forms red osmates upon reaction with a base, with ammonia, it forms the nitrido-osmates, etc.
Osmium
The most common compound exhibiting the +8 oxidation state is osmium tetroxide. This toxic compound is formed when powdered osmium is exposed to air. It is a very volatile, water-soluble, pale yellow, crystalline solid with a strong smell. Osmium powder has the characteristic smell of osmium tetroxide. Osmium tetroxide forms red osmates upon reaction with a base. With ammonia, it forms the nitrido-osmates . Osmium tetroxide boils at 130 °C and is a powerful oxidizing agent. By contrast, osmium dioxide (OsO) is black, non-volatile, and much less reactive and toxic.

the density of osmium which is the densest metal It is approximately twice as dense as lead and slightly denser than iridium. what is osmium? It is a chemical element with the symbol Os and atomic number 76. where did it get its name from? From Greek word  "osme" meaning "smell". who was the first one to discover numbers for atoms? Henry Moseley are there elements that are also missing? There were seven elements  which were not found and therefore identified as still undiscovered, but from 1918 to 1947, all seven of these missing elements were discovered what is the colour of the former chemical element? Bluish-white transition metal
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Osmium [SEP] Chemical properties
Osmium forms compounds with oxidation states ranging from −2 to +8. The most common oxidation states are +2, +3, +4, and +8. The +8 oxidation state is notable for being the highest attained by any chemical element aside from iridium's +9 and is encountered only in xenon, ruthenium, hassium, and iridium. The oxidation states −1 and −2 represented by the two reactive compounds and are used in the synthesis of osmium cluster compounds. The most common compound exhibiting the +8 oxidation state is osmium tetroxide. This toxic compound is formed when powdered osmium is exposed to air. It is a very volatile, water-soluble, pale yellow, crystalline solid with a strong smell.