Why is osmium denser than lead?

Many people are familiar with lead (11.3 kg/L), but osmium is twice as dense (22.6 kg/L)! Each liter (about 1/4 gallon) of osmium weighs 22.6 kg (50 lbs). ... Osmium and iridium are very dense materials ~22.6 g/cm3 -- about 20 times heavier than the same volume of water and about 10% more dense than plutonium. read more

It’s a matter a chemistry. Osmium’s nucleus may be less massive than Lead’s but their electron configurations allow Osmium’s atoms to be packed closer together than Lead. read more

Since both osmium and lead occupy the same amount of space in their crystal structures, there must be an additional reason for osmium being more dense despite its lower atomic mass. The final piece to the puzzle comes from the fact that osmium has a smaller atomic radius (1.34 angstroms) than lead (1.75 angstroms). read more

(The same with Osmium and Iridium). Even Mercury is denser than lead (13.6 mercury and 11 lead), but it's much more expensive and toxic. Some times those shields are made of concrete (they have to be much thicker than those made of lead), also to save money. read more