D-Block elements :
The elements in which last electron enters d-orbital of the penultimate (inner to ultimate) shell, i.e last electron goes to (n-1)d orbital are called d-block elements. D-block elements are also defined as those elements whose two outermost shell are incomplete (i.e partially field)
The general valance shell electronic configuration of d-block elements are also called transition elements. d-block elements is also called transition elements.
One of the most striking feature of the d-block elements is that they exihibit variety of oxidation states in their compound with the exception of a few elements most of the d-block elements show more than one oxidation state (i.e variable oxidation state). The following two reasons have been given to explain why d-block elements shows variable oxidation state.
(i) (n-1)d and ns orbitals in the atoms of d-block elements have almost the identical energy and hence electron can be removed from (n-1)d orbital as easily as they can be take part in different hybridization situation.
(ii) After removing ns electron (s) the remainder is called core or kernel. In case of most of d-block elements, the core is unstable and hence can lase one or more elements resulting in the formation of cation having different oxidation state i.e the removal of one or more electrons from the unstable core gives several oxidation state to transition metals.