Which halogen, bromine or iodine, will form the more polar bond with phosphorus?

By definition, electronegativity is the ability of an atom involved in a chemical bond to polarize electron density towards itself. There are various scales, of which the Pauling scale is the earliest, and is still widely.

It is well-known that electronegativity increases across a Period (i.e. it increases from left to right as we face the Periodic Table, but decreases down a Group). This makes sense on the basis of atomic structure: as we add protons to a nucleus, the nuclear charge attracts the valence electrons, and demonstrably reduces the atomic radius. On the other hand, when we descend a Group, the valence electrons are farther removed from the nuclear core, and the valence electrons are effectively shielded from the increased nuclear charge.

Iodine will form the more polar bond with phosphorus compared to bromine.