As discussed earlier, there may be one or more free electrons per atom which moves all the way through the interior of the metal under the influence of an applied field.
The following figure shows charge distribution within a metal. It is known as the electron-gas description of a metal.
The hashed region represents the nucleus with a positive charge. The blue dots represent the valence electrons in the outer shell of an atom. Basically, these electrons do not belong to any specific atom and as a result, they have lost their individual identity and roam freely atom to atom.
When the electrons are in an uninterrupted motion, the direction of transportation is changed at each collision with the heavy ions. This is based on electron-gas theory of a metal. The average distance between collisions is called the mean free path. The electrons, passing through a unit area, in the metal in the opposite direction in a given time, on a random basis, makes the average current zero.