Radical and Valence-Saturated Forms of Chemisorption

We see that the free electrons and holes of a crystal lattice are important factors in chemisorption and hence in catalysis. 

Both of them stand out as full and equal participants in the chemical processes involving chemisorbed particles. This is because, in these processes, as was shown in (2, 9-11), the free electrons and holes perform the functions of free valencies capable of breaking the valence bonds in the chemisorbed particles and themselves becoming saturated by these bonds. 

These functions of the free electrons and holes follow from the very concept of a free electron or a free hole. We shall illustrate this on the two limiting cases of a purely homopolar and a purely ionic crystal. 

As a typical example of an ionic crystal, consider a crystal of NaCl. The Na+ and Cl ions possess closed electron shells and in this sense are analogoues of the zero group atoms. A free electron in a NaCl crystal means that there is an extra electron attached to a Na+ ion outside the closed shell. Such an electron may be regarded as a free positive valence. A hole denotes that one of the Cl ions lacks an electron from its closed shell. Such a hole can therefore be regarded as a free negative valence. 

As another example, consider a CU2O crystal which we shall treat as an ionic crystal. As already observed, in this case a free electron corresponds to the state Cu and a free hole to the state Cu++, both of which wander among the normal Cu+ ions of the lattice. In the Cu atom and in the Cu+ and Cu++ ions, the electrons are distributed as follows  

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