Transition Elements – Introduction

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.

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Irregularities in Configurations

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