What is the significance of overlap in energy bands?

[Tsunami]

Homework Statement

I'm using http://ece-www.colorado.edu/~bart/book/" to understand my course on semiconductor physics better. In paragraph 2.3.4, the difference between insulators, semiconductors, metals is explained, based on the differences in bands. I understand the basic model.

What I don't understand, is figure 2.3.7 b) :

Materials consisting of atoms that contain two valence electrons can still be highly conducting if the resulting filled band overlaps with an empty band. This scenario is shown in b).

How should I visualize this? Which bands are overlapping? Are these two valence bands overlapping? (As is the case in most semiconductors) Or is this the overlapping of a valence band and a conduction band - hence, is there no bandgap then? Is this the behaviour of a metal?

Many thanks already,

tsu

 

[malawi_glenn]

I think it is the valence band that overlaps the conducting band, of the entire solid of course. These "bands" are for the solid as a whole, not for individual atoms.