This leaves a positive 'hole' in the valence band.
In metals, there are not enough electrons to fill the valence band completely.
The vacuum can be seen as a special type of insulator without a valence band.
Because the valence band is so nearly full, its electrons are not mobile, and cannot flow as electrical current.
The hole is an empty state that allows electrons in the valence band some degree of freedom.
Then, the density of free carriers in the conduction and/or the valence band will increase.
Hole conduction in a valence band can be explained by the following analogy.
This leaves an associated hole behind, in the valence band.
This is also applicable for holes in the valence band.
Most of the states with low energy (closer to the nucleus) are occupied, up to the valence band.
