Much confusion exists in the literature concerning the densities of osmium and iridium. This has occurred because these values are calculated from crystallographic data, and errors in the absolute value of the X-ray wavelength scale, Avogadro’s Number, and the atomic weights of these elements have only slowly been resolved. Unfortunately density values have been published from time to time which have incorporated one or more of these errors, and these have become fixed in the literature and repeated ever since.

The introduction of new values for Avogadro’s Number and the X-ray wavelength scale conversion factor (1), both of which have been used in the present calculations, is an apt moment to review the crystallographic data for these elements and to show that osmium is the densest metal. Avogadro’s Number is now (6.0221367 ± 0.0000036) × 10²³ per mol while the conversion factor from kX units to Ångström units is 1.00207789 ± 0.00000070. The equivalent Cu Kα ₁ X-ray wavelength standard is then 1.5405945 ± 0.0000011 Å and an earlier version of this value (1.540598 Å) has already been adopted by the U.S. National Bureau of Standards (2) as a wavelength standard to replace the currently accepted conversion factor of 1.00202.

For iridium, which has a face-centred cubic structure, the density is calculated from:

where A_r = atomic weight

The atomic weight is well established at 192.22 ± 0.03 (3) while an average lattice parameter of 3.8392 ± 0.0005 Å at 20°C is selected from the high precision measurements given in Table I, which have been corrected to this temperature using a thermal expansion coefficient of 6.4 × 10⁻⁶ per °C (4). The calculated density is 22.562 ± 0.009 g/cm³ and the selected value is then 22.56 g/cm³ (22,560 kg/m³).

For osmium, which has a closer-packed hexagonal structure, the density is calculated from:

where a and c = lattice parameters parallel to the a- and c-axes, respectively.

The accepted value for the atomic weight is 190.2 ± 0.1 (3), but this value is based entirely on the isotopic abundance measurements of Nier (10) and the actual value obtained is 190.238 ± 0.005 (11). Therefore 190.24 ± 0.10 has been used in calculating the density. Lattice parameters of a = 2.7343 ± 0.0005 Å and c = 4.3200 ± 0.0005 Å, at 20°C, have been selected from the measurements in Table II which have been corrected to this temperature using thermal expansion coefficients of 4.3 × 10⁻⁶ per °C parallel to the a-axis, and 6.8 × 10⁻⁶ per °C parallel to the c-axis (4).

The discrepant values of Owen, Pickup and Roberts, and Owen and Roberts may be associated with the relatively low purity of the metal used (99.8 per cent) while the measurements of Finkel’, Palatnik and Kovtun, although apparently carried out on pure single crystals, nevertheless differ significantly from all other determinations on high purity materials. Hence these three values were rejected and the remainder were averaged to obtain the selected lattice parameters. A density of 22.588 ± 0.015 g/cm³ is then calculated; inclusion of all lattice parameters would lower this value by only 0.003 g/cm³ however, and would therefore not affect the selected value of 22.59 g/cm³ (22,590 kg/m³) significantly.

Crystallographic data for the platinum group metals are presented in Table III. The lattice parameters for osmium and iridium are those selected above, while values for the remaining four platinum group metals are those selected by Donohue (17), after correction to the new conversion factor (1).

Having reviewed crystallographic data for both osmium and iridium, selected values of their densities at a temperature of 20°C are 22,590 kg/m³ and 22,560 kg/m³, respectively, thus confirming that osmium is the densest metal.