Abstract
The majority of metals show a linear relation between the heat of vaporization of the solid, the dissociation enthalpy of the dimer molecule and the product of solid surface energy and molar surface area. The divalent metals Zn, Cd, Hg and Mg are clear exceptions: their dimer molecules show unusually weak bonding. We suggest that for these divalent metals it makes sense to distinguish between two extreme situations, i.e. the metallic and the Van der Waals type modification. Which modification will prevail depends on both lattice dimensionality and pressure (at T = 0). It is argued that chains, free monolayers and small particles of Hg must be expected to have a dominant non-metallic character. For Cd and Zn a monolayer will be predominantly metallic, whereas a linear chain will be Van der Waals like. In Ca, Sr and Ba only the dimer can be of the Van der Waals type.
A number of problems are discussed in which the exceptional properties of divalent metals play a role (small particles, surface layers, sticking coefficients, metal–inert gas mixtures).