Why Thorium(At.Mass=90) which doesn't have any electron in its outermost f-orbital and even though it is placed in the f-block?

The expected electronic configuration of Th is [Rn]5f¹ 6d¹ 7s²  

however, the observed electronic configuration of Th is [Rn]5f⁰6d²7s²

So, Th has no electron in 5f orbital and 2 electrons in the 6d orbital because empty,half filled,and completely filled f orbital is more stable than any other configuration  . Thus, Th acquires this configuration to increase its stability, however it is a member of Actinide serise.