Adsorption, the hard-soft concept

Pearson looked into the stability between Lewis acids (electron acceptors) and Lewis bases (electron donors) and found that a stable complex forms between a hard acid and a hard base or between a soft acid and a soft base. 

For the central ion of a complex the character of the electronic shell is connected with the nature of the atomic orbital. The s, p, d, and f electrons are characterized by an increasing distance to the kernel and therefore can be seen as increasingly polarizable electrons. In the language of Pearson the character of these electrons varies from extremely hard (s-electrons) to extremely soft (f-electrons). The hardness increases with ionization and for different valance states with an increased oxidation state. 

One way to measure softness and hardness of molecules is the polarizability of the electron shell. A first criterion is the number of a- and jr-electrons. With an increasing number of 7i-electrons, the polarizability and the soft character usually increase. 

The Pearson concept can be transferred to metal surfaces. In this case, in addition to the character of the bonding electrons, the crystallographic structure of the metal surface and the surface topography must be taken into account. For cubic close-packed structures the most prominent crystallographic surfaces are the (111), (100), and (110) interfaces. The (111) face is the closest packed structure and has the hardest surface topography followed by the (100) and (110) interfaces (the softest one). The hard-soft character of a polycrystalline 

With increasing step density the softness increases. Furthermore, the softness increases with the surface density of kink site positions, or, quite generally, with increasing surface roughness. On polycrystalline surfaces, the same rules apply and the softness increases with an increasing amount of surface roughness. 

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