Final answer:
Calcium (Ca) has a larger atomic radius than selenium (Se) because it is further to the left and below Se in the periodic table, indicating an increase in atomic radius as you move down a group and a decrease as you move across a period.
Step-by-step explanation:
To determine which element has a larger atomic radius between calcium (Ca) and selenium (Se), one can refer to the periodic table. The atomic radius tends to increase as you move down a group and decrease as you move across a period from left to right. Calcium is located in group 2 (alkaline earth metals) and selenium is in group 16 (chalcogens), both in period 4. Since Ca is further to the left and below Se in the periodic table, Ca will have a larger atomic radius than Se. This can be explained by the fact that moving to the right across a period, effective nuclear charge increases drawing the electrons closer to the nucleus, thus decreasing the atomic radius. Additionally, as you move down a group, each element has an additional electron shell compared to the one above it, increasing the atomic radius despite the increase in nuclear charge.