The atomic radius generally decreases across a period (row) in the periodic table from left to right. This is because the effective nuclear charge (the net positive charge experienced by the electrons in the outermost shell) increases across a period. The increased effective nuclear charge draws the electrons closer to the nucleus, resulting in a decrease in the atomic radius.

In the case of F and Cl, F is located on the left side of the periodic table in period 2, while Cl is located on the right side of the periodic table in period 3. Therefore, F has a larger atomic radius than Cl.