The Osmium Day Thread

Here’s a question that, according to anecdotal stories on the internet, smart people get wrong all the time: Which is heavier, a pound of lead or a pound of feathers?

The correct answer is that they weigh the same. The stumbling block in people’s minds is that lead “weighs” more, but that’s not true. A pound is a pound. What they’re thinking is that a given volume of lead weighs more, and they’re mistaking weight for density. If you took two identical shoeboxes and filled one with lead and the other with feathers, the box filled with lead would indeed be heavier. But that’s not the heaviest you could make it.

Lead isn’t the densest element – it isn’t even in the top 10. There’s an element that’s almost twice as dense, and that element is osmium.


Osmium is a platinum group metal, one of six. The platinum group metals are (generally speaking), hard, dense, and rare. (Three of the others – iridium, platinum and rhenium – are the second-, third-, and fourth-densest elements.)

In an earlier age, these characteristics made osmium good for pen nibs and phonograph needles. Or at least it sounded good for phonograph needs – element collector Theo Gray has yet to have any luck finding a needle that really has osmium in it.


Unusually for a platinum-group metal, osmium forms an oxide – osmium tetroxide – when exposed to air. Worse, the oxide is extremely toxic. But it does have industrial applications, which will turn out to be the case for pretty much every element I make a thread about. No matter how rare, no matter how toxic, industry will find a use for every element, as long as it isn’t too radioactive.