How can the free charges of a conductor redistribute to the surface? 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'?
 A: Think of it a different way.
The electrons from only the atoms on the surface of the metal redistribute themselves.
Part of the surface layer becomes positive net, and part of the surface layer becomes negative net.
The net charge of the entire surface remains zero.
A: When the metallic bond is formed metal atoms lose electrons to form positive ions which are fixed in a lattice and a sea of mobile (free) electrons which are not bound to an particular nucleus and are responsible for the conduction process.  
When subjected to an external electric field these free electrons redistribute themselves so that there is no electric field inside the conductor.  
A region to which some of the mobile electrons have migrate to has a surplus of electrons and so is designated negative whereas a region which some of the free electrons have left has a deficit of electrons and so is designated positive.  
The net charge on the conductor is zero as for every electron which moves to one region it leaves a positive charge of the came magnitude in the region that it has left.
