What is the densest naturally occurring element?

Osmium, at about 22.6 grams per cubic centimetre - roughly twice the density of lead, so a litre bottle filled with it would weigh 22 kilograms. Iridium is a fraction behind and the two swapped places repeatedly as measurements improved, since both are difficult to obtain pure enough to weigh accurately.

The density comes from the same relativistic effects that make mercury liquid and gold yellow - electrons in heavy atoms move fast enough that their effective mass increases and their orbits contract, packing the atoms tighter. It sits in the platinum group, and it is brittle, hard, and has the unpleasant property of forming a volatile oxide that damages lungs and eyes at very low concentrations, which is where the name comes from - osme, Greek for smell.

Its main use is where extreme hardness and wear resistance matter in something small - fountain pen nib tips, instrument pivots, electrical contacts. It is also the material of the smallest known example of an intriguing engineering problem: an osmium sphere the size of a football would be too heavy for one person to lift, which makes it a favourite thought experiment and an expensive one to test.