Osmium (Os) is one of those elements situated smack dab in the middle of the periodic table and thus destined to be ignored by pretty much everyone. Let's show it a little love, shall we? A couple of choice facts about osmium: it is a member of the platinum family of transition metals, is incredibly dense (22.61 g/cm3 - gold is only 19.3 g/cm3 and lead a measly 11.34 g/cm3), has a very high melting point (3033 ºC - iron melts at 1538 ºC), and gets its name from the Greek osme (meaning odour). To explain that last bit, when solid osmium is heated above 100 C, it readily reacts with oxygen in the air to form osmium tetroxide, which apparently stinks like ozone or chlorine (i.e. very acrid). This reaction occurs slowly at room temperature with powdered osmium.
Osmium tetroxide is a wonderful combination of highly volatile (evaporates quite readily) and highly toxic. Solids generally don't appreciably sublime (convert directly into a gas) at room temperature, but osmium tetroxide is special. Its high toxicity is the result of it being: (A) an extremely powerful oxidizing agent, meaning that it loves to react with (and so damage) living tissues, and (B) relatively small and water soluble, such that it easily penetrates skin and mucous membranes.
Exposure to even low concentrations of osmium tetroxide vapour can cause severe irritation of the skin, eyes, and respiratory tract. This can mean fun things like dermatitis and ulceration, blindness (it can stain the cornea of the eye), and pulmonary edema. Chronic exposure to the vapour can lead to the accumulation of osmium in the liver and kidney with damage to these organs.
The whole oxidizing living tissues things makes osmium tetroxide useful to researchers as a means of fixing biological samples in place, since the oxidation of macromolecules such as lipids leads to their atoms becoming cross-linked. It is also used by researchers to stain biological things in order to image them using transmission electron microscopy. Osmium nuclei are nice and dense, and so are good at scattering electrons.
- Makarovsky I, Markel G, Hoffman A, Schein O, Finkelstien A, Brosh-Nissimov T, Tashma Z, Dushnitsky T, Eisenkraft A. Osmium tetroxide: a new kind of weapon. Isr Med Assoc J. 2007 Oct;9(10):750-2. Review.
- Smith IC, Carson BL, Ferguson TL. Osmium: an appraisal of environmental exposure. Environ Health Perspect. 1974 Aug;8:201-13.