* Electronegativity: Fluorine (F) is the most electronegative element on the periodic table. Chlorine (Cl) is also electronegative, but less so than fluorine.

* Polarity: Electronegativity difference between two atoms in a bond determines its polarity. A larger difference leads to a more polar bond.

Therefore, the P-F bond in PF3 is more polar than the P-Cl bond in PCl3 because the electronegativity difference between phosphorus and fluorine is greater than the difference between phosphorus and chlorine.