d-Block elements occupy the middle portion of the periodic table - between s- and p- block elements. They include elements from groups 3 to 12. In these elements the outermost shell contains one or two electrons in their outer most ns orbital but the last electron enters into the inner d-subshell - (n-l) d orbital.
The elements of the d-block are metallic in nature. Their general characteristic properties are intermediate between those of the s block elements, on one hand and of the p-block elements on the other.
Transition elements are elements in which the d subshell is partially filled either in atomic state or in ionic state.
There are four series of transition elements in the periodic table. The first transition series begins with scandium (At. No. 21) and ends at copper (At. No. 29) whereas the second, third and fourth series begin with yttrium (At. No. 39), lanthanum (At. No. 57) and actinium (At. No. 89) and end at silver (At. No. 47), gold (At. No. 79) and at the element having atomic number 112 (a synthetic element), respectively. These series are also referred to as 3d, 4d, 5d and 6d series, respectively.
Zinc, cadmium and mercury do not have partially filled d subshell either in the elemental state or in any of their common ions. Therefore, these elements are not transition elements. However, zinc, cadmium and mercury are considered along with d- block elements.
The general electronic configuration of transition elements is (n-1)d1-10ns1-2. The (n-1) stands for inner shell and the d-orbitals may have one to ten electrons and the s-orbital of the outermost shell (n) may have one or two electrons.
After filling of 4s orbital successively with two electrons at atomic number 19 and 20, the next incoming electron goes to 3d orbital instead of 4p, as the 3d orbital is of lower energy than the 4p orbital. In the case of next nine elements following calcium, the incoming electron is filled in the d- subshell.
Since half filled and completely filled subshells are more stable than the one in which one electron is short, an electron gets transferred from 4s to 3d in case of the elements.
Ionization (Oxidation) of Transition Elements
During ionization electrons should be lost first from the (n-1)d subshells and then from the 4s level. However, this is not the case. The reason for the deviation from the expected behavior is that once the filling of the 3d subshell commences at scandium (At. No.21) energy of 3d subshell decreases and becomes lower than that of 4s subshell. Consequently, on ionization, the first row transition elements lose electrons from the 4s subshell followed by the loss from 3d level.