The best explanation for why the atomic radius of one element is greater than that of another typically relates to periodic trends in the periodic table. Atomic radius increases as you move down a group because each new row adds a new electron shell, which increases the distance of the outermost electrons from the nucleus. Also, atomic radius decreases as you move from left to right across a period because increasing nuclear charge pulls electrons closer to the nucleus. So, if the atomic radius of an element (like potassium, K) is greater than that of another element (like bromine, Br), it is because potassium is located to the left and above bromine on the periodic table, meaning potassium has fewer protons attracting the electrons and has fewer electron shells, resulting in a larger atomic radius. The shielding effect and nuclear charge differences explain this difference in size best. In summary, the best explanation for a larger atomic radius in one element versus another is that the element with the larger radius has either more electron shells (is lower on the periodic table) or has a lower effective nuclear charge pulling the electrons closer (more to the left in a period).