Transition elements– Elements which involve filling of d-orbitals in penalty made shell are called d-block elements or transition elements. In periodic table they are placed in between s block and p block elements and their properties represent a change from electropositive elements to electronegative elements, that is why they are commonly called transition elements.
Transition elements of d block may be defined as those elements which have partially filled d orbital in their elementary form or in their commonly occuring oxidation state. Zinc , Cadmium and Mercury are not regarded as transition elements because their subshell is completely filled.
The d block of periodic table consist of following transition series-
- First transition series – It is fourth period of periodic table. 3d orbitals are generally filled. It consist of element from Scandium (Atomic number- 21) to Zinc (30).
- Second transition series – It is 5th period of periodic table. 4d orbitals are generally filled. It consist of elements from Yttrium (39) to Cadmium (48).
- Third transition series – It is 6th period of periodic table. 5d orbitals are generally filled. It consist of elements from Hafnium (72) to Mercury (80).