Why does ferrous oxide is more basic than ferric Oxide?

Compare the Fe²⁺ ion in ferrous oxide and Fe³⁺ ion in ferric oxide.

Fe³⁺ ion carries a higher positive charge and is smaller in size. Therefore, Fe³⁺ ion has a stronger polarizing power to polarize the neighboring oxide ions. "Stronger polarizing power" means that Fe³⁺ ion attracts the electron clouds of neighboring oxide ions to a greater extent.

As the oxide ions in ferric oxide is polarized to a greater extent, the oxide ions in ferric oxide have less electron clouds to accept protons (hydrogen ions). Therefore, the oxide ions in ferric oxide are less basic than the oxide ions in ferrous oxide.