What are the transition elements?

Those elements, which have partially filled d or f-subshells in atomic state, are called transition elements. The d-block and s-block elements are called transition elements because they are located between the s and p block elements.

These elements are divided into two types.

Outer transition elements

Inner  transition elements

Outer Transition Elements:

Those elements having partially filled d-subshells are called outer transition elements or d-block elements. Cadmium, Zinc,  and mercury are also included in the transition elements, although they  have no partially filled d-orbital.