Question

why charge given to conductor resides over its surface

Answer 1

In a static situation there are no excess charges inside a conductor and the net electric field is zero due to that. This is because the free charges have distributed themselves in such a manner that the resultant effect is cancelled out and is zero.

So, its only on the surface of a conductor that there is a net effect due to excess charges and thus electric field is not zero.

Thus, all excess charges exits on a conductor's surface.