* Electronegativity: Fluorine is the most electronegative element on the periodic table, meaning it has a strong attraction for electrons. Chlorine is also electronegative but less so than fluorine.

* Difference in Electronegativity: The greater the difference in electronegativity between two atoms in a bond, the more polar the bond. The difference in electronegativity between phosphorus and fluorine is significantly larger than the difference between phosphorus and chlorine.

In summary: Because fluorine is much more electronegative than chlorine, the P-F bond has a much larger partial positive charge on the phosphorus atom and a much larger partial negative charge on the fluorine atom, making it more polar.