Final answer:
Moving from left to right across a period, there is a greater increase in effective nuclear charge compared to moving from top to bottom in a group.
Step-by-step explanation:
As we move from left to right across a period on the periodic table, there is a greater increase in effective nuclear charge compared to moving from top to bottom in a group. This is because the number of protons in the nucleus increases from left to right, resulting in a stronger attraction between the nucleus and the electrons. The increase in nuclear charge pulls the electrons closer to the nucleus, decreasing the atomic radius.