Question

What are inner and outer transitional elements?

Answer 1

The current IUPAC definition states that a transition element is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell."
However, a lot of people also use "transition element" to refer to elements of both the d-block and the f-block. In this case, to distinguish between the two blocks, the d-block is called "outer transition elements" and the f-block is called "inner transition elements".