osmium (Os)Article Free Pass osmium (Os), chemical element, one
of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6,
of the periodic table and the densest naturally occurring
element. A gray-white metal, osmium is very hard, brittle, and
difficult to work, even at high temperatures. Of the platinum
metals it has the highest melting point, so fusing and casting
are difficult. Osmium wires were used for filaments of early
incandescent lamps before the introduction of tungsten. It has
been used chiefly as a hardener in alloys of the platinum
metals, though ruthenium has generally replaced it. A hard alloy
of osmium and iridium has been used for tips of fountain pens
and phonograph needles, and osmium tetroxide is used in certain
organic syntheses. Pure osmium metal does not occur in nature.
Osmium has a low crustal abundance of about 0.001 part per
million. Though rare, osmium is found in native alloys with
other platinum metals: in siserskite (up to 80 percent), in
iridosmine, in aurosmiridium (25 pe