I got the concept of spin quantum number and got enlightened of the fact that it has got nothing to do with the movement of electron around the nucleus and that it is concerned only with the rotation of electrons about their own axis. I thank 'pf mentor' for giving me clarity about that. But I still dont get why two electrons present in the same orbital dont collide each other.. I didn't understand the example of surface of a ball and orbital. So plz dont mind clarifying my doubt..