Definition and Electronic Configurations of Atoms The elements lying between s and p-block elements of the periodic table are collectively known as transition or transitional elements (T.E.’s):
These elements either in their atomic state or in any of their common oxidation state have partly filled (n-1) d-orbitals of (n-1)th main shell.
In these elements the differentiating electron enters (n-1)d orbitals of (n-1)th main shell and as such these are called d-block elements.
The valence shell configurations of these elements can be represented by (n-1)d1-10.ns0 , 1 , 2. The configurations clearly show that strictly, according to the definition of d-block elements, Cu, Ag and Au should be excluded from d-block elements, since these elements, both in their atomic state [with configuration (n-1)d10ns1] and in their +1 oxidation state [with configuration (n-1)d10], do not have partly filled (n – 1)d-orbitals.
Similarly Zn, Cd and Hg which both in their atomic state [(n-1)d10ns2 ] and in +2 oxidation [(n-1)d10] do not contain partly filled (n-1)d orbitals, should also be excluded from d-block elements.
Similar is the case with Pd atom with configuration 4d105s0. Yet, in order to maintain a rational classification of elements, these elements (viz Cu, Ag, Au, Zn, Cd, Hg and Pd) are also generally studied with d-block elements.
All the d-block elements are classified into four series viz 3d, 4d, 5d and 6d series corresponding to the filling of 3d, 4d, 5d and 6d orbitals of (n-1)th main shell. Each of 3d, 4d and 5d series has ten elements while 6d series has at present only one element viz Ac80 whose valence shell configuration is 6d1 7s2.
Irregularities in Configurations
The irregularities in the observed configurations of Cr (3d54s1), Cu (3d104s1), Mo (4d55s1), Pd (4d105s0), Ag (4d10 5s1) and Au (5d106s1) are explained on the basis of the concept that half-filled and completely filled d-orbitals are relatively more stable than other d-orbitals.
On the basis of the above concept it is, however, not easy to explain the irregularities found in the observed electronic configurations of the atoms of other elements, since one has to consider the net effect of so many other factors such as
(i) nuclear electronic attraction
(ii) shielding of one electron by several other electrons
(iii) inter-electronic repulsion
(iv) the exchange-energy forces etc.
All these factors play an important part together in determining the final stability of an electronic configuration of an atom. It is not easy to explain why W unlike Cr(3d55s1) and Mo(4d5 5s1) should have the idealised electronic configuration (4f145d46s2).
The properties of transition elements of any given period are not so much different from one another as those of the same period of non-transition elements. The reason of this fact lies in the electronic configuration of transition elements.
We know that electronic configurations of transition elements is invariably (n-1)d1–10 ns0 or 1 or 2 which indicates that
(i) the electronic configurations of transition elements differ from one another only in the number of electrons in d orbitals in the (n-1)th shell and
(ii) the number of electrons in the outermost shell, ns, is invariably 1 or 2.
Transition Elements : Introduction
Formation of Alloys