lanthanoids form primarily +3 ions, while the actinoids usually have higher oxidation states in their compounds, +4 or even +6 being typical. why?

Lanthanides show variable oxidation states. The most stable oxidation state of Lanthanides is +3. They also show +2 and +4 oxidation states due to presence of either half filled or completely filled or empty 4f sub shell. Unlike lanthanides, actinides show a variety of oxidation states from +3 to +6 due to the very small energy gap between 5f, 6d and 7s sub shells. The principal oxidation states are +3 and +4. The +3 oxidation state is the most stable. The +4 oxidation state is the most stable in Th and Pu. +5 in Pa and Np and +6 is seen in U. In actinides, the distributions of oxidation states are so uneven.