Final answer:
Among S, Cl, Se, and Br, chlorine (Cl) has the smallest atomic radius because it is located to the right of sulfur (S) in the periodic table and atomic radii decrease across a period.
Step-by-step explanation:
The size of an atom's radius typically decreases from left to right across a period in the periodic table and increases as you move down a group. Within the periodic table, sulfur (S), chlorine (Cl), selenium (Se), and bromine (Br) are found as such:
- S and Cl are in the third period with Cl being to the right of S.
- Se and Br are in the fourth period with Br being to the right of Se.
Considering these positions, chlorine (Cl) has a smaller atomic radius than sulfur (S) because it is to the right of sulfur in the same period, which generally indicates a smaller radius due to increased nuclear charge pulling the electrons closer to the nucleus. Selenium (Se), being below sulfur in the periodic table, will have a larger atomic radius than sulfur because it is in the next period down. Similarly, bromine (Br), being below chlorine and to the right within the same period as selenium, will also have a larger atomic radius than chlorine.
Hence, within this group, chlorine (Cl) has the smallest atomic radius.