Read the passage given below and answer the following questions: 

The f-block of the periodic table consists of two series of elements: Lanthanoids and actinoids with the successive filling of the inner orbitals (4f or 5f), there is a gradual decrease in the atomic and ionic size of these metals along the series.

The principal oxidation state of lanthanoids is +3, although +2 and +4 oxidation states are also exhibited by some lanthanoids. The chemistry of actinoids is more complex in view of their ability to show variable oxidation states and their radioactive nature.