At the most basic level, density is determined by how close the atoms or molecules in a substance are packed, as well as the mass of those atoms. While it’s not quite as simple as assuming that the heaviest elements on the periodic table have the highest densities, having a high mass does help: heavy metals like iridium and osmium are the densest metals known to date. Remember that the density of a material does not depend on how much material you have; the density of 1 gram of lead is the same as the density of 1 kilogram of lead. Density is an intensive property, so changing the amount of a material you have does not affect its density.