There are a number of instances where the assumption of a weak potential gives an accurate description of electrons moving through a lattice, that is, the electrons behave almost as free electrons. There are two reasons why this is sometimes the case.
Firstly, the interaction of electrons with the ions is much stronger at small separations, the conduction electrons are separated from the ions by the core electrons that surround it, and so the interaction is weak. Secondly, in this region in which the conduction electrons are allowed, their mobility can have the effect of screening the fields of the ions, making the potential experienced by any given conduction electron very weak.
It means that the connection between coeducation electrons with the core electron is much stronger than the connection between these electrons with ions.
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