Excitation and fluorescence spectra of group IIB atoms (Hg, Cd, Zn) have been extensively measured in the last years. From these measurements dependable values for potential parameters like well-depth and vibrational frequency for the ground electronic state have been derived with a high degree of accuracy. A determination of the equilibrium distance for Hg is also available. For Cd and Zn, the equilibrium distances are not known, though. Here we propose to use low-density viscosity measurements together with the spectroscopic data to give an estimate of these distances.