In the later actinides, why does the +3 state become predominent?
The term oxidation state refers to the number of electron(s) that are involved or that can possibly become involved in the formation of chemical bond(s) in that compound, when one element combines with another element during a chemical reaction. All actinides are characterized by partially filled 5f, 6d, and 7s orbitals. In the early actinides the 5f orbitals are less core like. In addition, the 5f and 6d orbitals are more similar in energy. So that d orbital participation can increase the covalent contribution to bonding, allowing oxidation states above +4 to form. However as the atomic number increases the 5f orbitals become more core-like, bonding becomes more ionic and oxidation state +3, becomes more dominant (+3 is the most stable oxidation state for Am and all heavier actinides.).