I have studied that if I put a conductor (perfect or non-perfect) at rest in a place where there is an electrostatic field, the charges of the conductor will distribute so that negative charges will be in the surface near the source of the electric field, leaving positive charges on the other side.
I cannot understand this. If the conductor is idle, e.g. not connected to a battery, I assume there are no 'extra' electrons within its lattice. In that case, the number of free charges (electrons) will be such that the net charge of the metal is zero and if an electron moves from its nuclei to another, another electron will come to replace it, avoiding the creation of 'holes' (positive free charges). Unless there exist natural 'holes' in every metal, I cannot see how an accumulation of negative charge can appear on one side of the conductor generating a positive accumulation on the other side. How is it possible? Should not be 'extra' electrons (from a battery, from a capacitor...) be needed?
EDIT: Even in the case of 'extra' electrons, I cannot see how positive charge could appear on the other side. I understand that, in that situation, a negative accumulation of charge would be in the surface of the metal near the source of the field, but how could the positive zone of charge take place? Where is the space to allocate those 'holes'?