1. While going from left to right within a period, the atomic number increases one by one, meaning the positive charge on the nucleus increases by one unit at a time.
2. However, the additional electron gets added to the same outermost shell.
3. Due to the increased nuclear charge, the electrons are pulled towards the nucleus to a greater extent and there by the size of the atom decreases. Thus the atomic radius goes on decreasing while going from left to right in a period.