The d-block elements are located in the middle of the periodic table and consists of metals only. It consists of four series, each series consists of 10 elements.
In these elements, the last electron enters the d orbital of the penultimate shell i.e. the last electron goes to (n-1) d orbital. Hence these elements are named as d-block elements. These elements have partly filled d-subshells in their elementary form or in their simple ions. The d-block elements are called transition elements because these represent a transition from highly electropositive elements (metals) of s-block to least electropositive elements (non-metals) of p-block.
Classification of d-block Elements
Based on whether the last electron goes to 3d,4d,5d or 6d orbital, d-block
elements are classified into four series. They are
i) 3d series or First transition series (21Sc to 30Zn)
ii) 4d series or Second transition series (39Y to 48Cd)
iii) 5d series or Third transition series (57La and 72Hf to 80Hg)
iv) 6d series or Fourth transition series (89Ac and 104Rf to 112) or Incomplete series.
Electronic configuration of d-block Elements
In the transition elements, d-orbitals of penultimate shell are successively filled. The first transition series involves the filling of 3d orbitals. It starts from scandium (Z=21) and goes up to zinc (Z=30).
The second transition series involves the filling of 4d-orbitals and includes 10 elements from yttrium (Z=39) to cadmium (Z=48).
The third transition series involves filling of 5d-orbitals. The first element of this series is lanthanum (Z=57). It is followed by fourteen elements called lanthanides which involve the filling of 4f-orbitals. The next nine elements from hafnium (Z=72) to mercury (Z=80) belong to third transition series.
The general electronic configuration of transition elements is (n-1)d1-10 ns1-2.