Explanation:
As you move across the periodic table, the number of protons and neutrons increases but the number of orbital levels of the period remains the same. The atomic radii therefore decrease, across the period, because the increase in proton number causes an increased pull of the orbital electrons bringing them closer to the nucleus.
As you move down a group in a periodic table, the number of orbital levels increase. The effective nuclear charge of the nucleus of the atoms decreases due to the increased number of orbital levels that shield the valence electrons from the attractive force nucleus.
