Until recently much confusion existed in the literature as to which is the densest metal, osmium or iridium. Crabtree reviewed the experimental data, but his calculated densities of 22.59 ± 0.02 g/cm³ and 22.57 ± 0.02 g/cm³, respectively, led to an overlap in the uncertainties and a suggestion that the problem had not been solved (1). However, in a similar review by the present author, calculated densities at 20°C of 22.588 ± 0.015 g/cm³ and 22.562 ± 0.009 g/cm³, respectively, suggested that the problem just may have been resolved in favour of osmium (2).

Although the uncertainties calculated for the densities are usually dominated by the accuracies assigned to the lattice parameters, the accuracy of the atomic weight can also contribute significantly – if it is poorly known, as was the case for osmium until quite recendy. However with newly accepted atomic weights of 190.23 ± 0.03 for osmium and 192.217 ± 0.003 for iridium the contribution to the uncertainties of the densities is now minor (3).

Using the lattice parameters selected previously by the present author (2), the calculated density of osmium is revised to 22.587 ± 0.009 g/cm³ while that for iridium remains the same. On these grounds it is suggested that osmium can now unambiguously be considered to be the densest metal at 20°C.