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The word “osmium” is unlikely to be the first word that comes to mind when you wake up after a good night’s sleep. And it is that osmium (Os) is quite rare, yes, but it has special properties that make it a fascinating element. To begin with, say that osmium derives from the Greek word osme , “smell”. At the end of the article we will understand why. It is in the center of the periodic table and has some very strange neighbors, so much so that some of them do not really exist in nature, and have been produced artificially in the laboratory (which is why they are called “synthetic elements”). But it also has valuable neighbors, with very good press and really admirable: palladium, silver, platinum and gold. Osmium is not far behind.
Thus, osmium is as rare as it is valuable. In fact, it is so rare that it is the least abundant element in the Earth’s crust. For each gram of osmium there are 307,333,333 grams of oxygen; but oxygen, O for his many friends who owe him so much, plays to the advantage, because it is the most abundant element.
Osmium is also the densest of all metallic elements , and by extension, of all elements. Its density of 22.6 g/ml makes it 22.6 times heavier than water, as expected. The densities of metals vary greatly: the lightest are at the top of the periodic table and the heaviest at the bottom. Here are some examples (in grams/ml):
- lithium 0.53
- sodium 0.97
- Potassium 0.89
- Iron 7.9
- Lead 11.3
- Mercury 13.5
- Gold 19.3
osmium density
The density of an element is related to the number of atoms of that element that can be placed in a given volume, and also to the weight of the nuclei of that element. Therefore, the smaller the atomic radius of an atom and the higher the atomic number of the nucleus, the higher the density of the element.
The small atomic radius of osmium results in a close separation between its atoms. This small atomic separation, together with osmium’s relatively high atomic number, explains its high density.
The size of the atomic radius can be attributed to the following factors, all of which are quantum in nature:
- The f orbitals are very diffuse and therefore result in poor filtering of the outermost electrons. In the case of osmium (whose external atomic structure is: 4f 14 5d 6 6s 2 ), the poor shielding of its 4f orbitals leads to a contraction of the n=5 and n=6 orbitals.
- Due to the high atomic number of osmium, relativistic effects come into play. Basically, in the case of heavy nuclei, or rather dense, the electrons, to remain stable in their orbit, must move at relativistic speeds (the relativistic speed is that which represents a significant percentage of the speed of light). Under such circumstances, the mass of those relativistic electrons increases and the radius of the s orbital decreases (the radius of the p orbital also decreases, but to a lesser extent).
- The orbital contraction caused by these two effects results in a much smaller atomic radius than would be expected for osmium. As a result, metal-metal bonds are short. This is reflected in the small volume of the unit cell of the metallic bonds of the osmium (27.96 cubic angstroms). For comparison, the volume of the unit cell of lead is 121.3 cubic angstroms. Therefore, many more osmium atoms can be packed into a given volume than atoms of other elements.
- Osmium’s relatively high atomic number, coupled with its small atomic radius, as explained above, gives rise to osmium’s high density.
What is osmium used for?
Due to its chemical stability, durability, and hardness, osmium is used to make electrical contacts, phonograph styli, fountain pens, and jewelry. But things change drastically when combined with four oxygen atoms: you get a totally different kind of chemical beast, osmium tetroxide, which, in addition to being highly dangerous to your health when inhaled, has up to 50 shades of olfactory disgust. In other words, it leaves you in the dark and on top of that it smells terrible, much worse than we can imagine. However, some organic chemists, also more than we can imagine, use it for an extraordinarily selfish reason: to convert an alkene (hydrocarbon with one carbon-carbon double bond) into a diol (hydrocarbon with two alcohol groups, that is! , OH)!. And it is already known:
