Osmium

Osmium edges out iridium as the densest element — 22.59 against 22.56 g/cm³, a margin only settled by X-ray crystallography rather than weighing — so a litre of it would mass 22.6 kg. Its name comes from Greek osme, smell, for osmium tetroxide, a volatile solid whose vapour attacks the cornea and can cause permanent blindness at concentrations too low to be obviously dangerous. That same tetroxide is indispensable in biology, where it fixes and stains lipid membranes for electron microscopy by binding to unsaturated bonds. Alloyed with iridium it makes osmiridium, the traditional tip of a fountain pen nib and of early phonograph styli, chosen because it essentially does not wear.