The series of elements, that are formed by filling the 3d, 4d and 5d shells of electrons, comprise the d-block elements. They are often called as transition elements because their position in the periodic table is between s-block and p-block elements. Their properties are transitional between the highly reactive metallic elements of the s-block, which form ionic compounds and elements of p-block which form covalent compounds. In s and p blocks electrons add to the last shell, in d-block electrons are added to the penultimate shell. Typically the transition elements have an incompletely filled d level. The zinc group has d10 configuration and compounds of these elements show some differences from other transition elements. The elements make up three complete rows of ten elements and an incomplete fourth row. The position of the incomplete fourth series is discussed along with the f-block elements.
Illustration 1. Through copper, silver and fold have completely filled sets of d-orbitals yet they are considered as transition metals. Why?
Solution: These metals in their common oxidation states have incompletely filled d-orbitals e.g. Cu2+ has 3d9 and Au3+ has 5d8 configurations.
Illustration 2. Zinc, cadmium and mercury are generally not considered as transition metals. Give reasons. Solution: These elements in their most common oxidation state of +2 have completely filled d-orbitals.
The general electronic configuration is (n – 1)1-10 ns1-2where n is the outermost shell. The number of electrons in their outermost subshell remains two while their penultimate shell of electrons expands from 8 to 18 electrons.
Electronic configuration of 3d series
Anamolous configuration of Cu and Cr
Copper and chromium have a single electron in 4s-orbital. This is due to the gain of additional stability by the atom having either half-filled (5 electrons) or completely filled (10 electrons) d-shell.
Illustration 3. In what way is the electronic configuration of transition elements different form that of the non-transition elements?
Solution: Transition elements contain incompletely filled d-subshell i.e. their electronic configuration is (n – 1)d1-10ns0-2 whereas non-transition elements have no dsubshell or their subshell is completely filled and have ns1-2np1-2 in their outermost shell.
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