Final answer:
As the mass of an atom increases, typically so does the atomic radius, primarily seen as one moves down a column in the periodic table due to added electron shells. Across a periodic table, however, the radius tends to decrease as mass increases because of the stronger pull of an increasing number of protons on the electron cloud.
Step-by-step explanation:
When the mass of an atom gets larger, typically the radius of the atom increases as well. This is seen clearly as you move down a column of the periodic table, where the atomic radii increase due to the addition of more electron shells. The valence electron shell gets larger, and with a larger principal quantum number, the valence shell is physically farther from the nucleus. Furthermore, higher principal energy levels consist of orbitals which are larger in size. Although increasing nuclear charge could tighten the electron cloud, the addition of electron shells is the dominant factor, leading to an increase in atomic radius as the atom's mass increases.
However, this trend changes across a period; as you move from left to right across a periodic table, the atomic radius generally decreases. This is due to an increase in the positive charge from added protons pulling electrons closer to the nucleus, despite these electrons being added to the same principal energy level. Here the increase in nuclear charge outweighs the addition of electron mass, causing a decrease in atomic radius.