I've been told that electricity is due to flow of charges(-ve charge=electron) that are provided by the battery. And no. of charges that leave battery at one end are equal to no. of charges that re-enter battery. Question arises in my mind that when electrons leave the battery and enter the conductor then don't they interfere with the valance shell electrons of conductor? And if they travel through the conductor the valance electrons of conductor atoms should also start travelling with them ?
It's not entirely appropriate to consider the electrons of a conductor to be in a valence shell. Rather, in what is termed a "sea of electrons", you can think of them moving freely in the region around the atoms, not bound to any single one in particular.
Now, the common analogy is to think of this electron sea as a garden hose filled with water. If you put water in one end, you will get water coming out the other. However, the water you put in doesn't immediately travel the full length of the hose. Instead, it pushes out water that was already close to the end.
In reality, electrons coming out of a battery move at a speed that is incredibly slow (according to wikipedia, millimeters per hour). However, when you hook up a light-bulb to a battery, it turns on almost instantly. This is because, like a garden hose, the battery pushes electrons that are already in the conductor
So, in short, not only do the electrons coming out of a battery "interfere" with the electrons of the conductor, but this is actually the reason that electricity can "travel" so quickly in a wire.