Final answer:
The atomic radius generally increases as you move down a family on the periodic table. This increase is due to the addition of electron shells that provide greater distance and shielding from the nucleus despite an increasing positive nuclear charge.
Step-by-step explanation:
The trend in atomic radius as you move down a family on the periodic table, according to Coulomb's law, is that the atomic radius generally increases. This is because as you go down a group, each subsequent element has an additional electron shell, resulting in a larger principal quantum number. Consequently, the valence electron shell is physically farther away from the nucleus despite the increase in positive nuclear charge. The increasing distance outweighs the nuclear attraction due to the additional inner electron shells that also provide a shielding effect. Therefore, the atomic radius increases down a group.
Within a group like the alkali metals, as the atomic number (Z) increases, there is also an increase in the positive nuclear charge. However, because there is a greater number of occupied principal energy levels with larger orbitals as we go down the group, the overall effect is that atomic radius increases with an increase in atomic number down the group.