In general, smaller cations have higher polarizing power because they have a higher charge density, meaning the positive charge is concentrated in a smaller volume. Therefore, Al3+ has higher polarizing power compared to Fe3+.
Aluminum (Al) has an atomic number of 13 and forms the Al3+ cation by losing three electrons. Iron (Fe), on the other hand, has an atomic number of 26 and forms the Fe3+ cation by losing three electrons. The ionic radius of Al3+ (0.54 Å) is smaller than that of Fe3+ (0.64 Å).
Due to its smaller size, the Al3+ cation can come closer to an anion and exert a stronger attraction on its electrons, resulting in higher polarizing power. This ability to distort the electron cloud of an anion is important in various chemical reactions and coordination complexes.
It's worth noting that polarizing power is just one factor among many that influence chemical behavior, and different ions can exhibit different reactivities depending on the specific circumstances and other factors involved.