Transition Elements and its Classification

Transition Elements:

These are also known as d-block elements (group 3 – 12) because the last electron in these elements enter in d-orbital of the Penultimate (inner to the outermost) shell and are defined as, “those elements which have partially filled (n-1)d sub-shell either in the elementary state or in any of their common oxidation states”. However, this definition does not include transition metals like Zn, Cd and Hg because they do not contain a partially filled d sub-shell either in the elementary state or in their oxidation states, but due to some similar properties, they are included in Transition elements. These elements are placed between s-block and p-block in the Periodic table and, hence, are known as Transition elements, because they show a transition in their properties from more electropositive elements on their left (s-block) to the more electronegative elements on their right (p-block). Their general outer electronic configuration is (n-1) d^1-10 ns^0-2, where ‘n’ represents an outermost shell.

Classification of Transition Elements:

Transition elements can be classified into Four series depending upon the d sub-shell in which the last electron enters. These four series of transition elements are as under-

• First Transition Series or 3d series (4th period)- This series involves the filling of 3d sub-shell and consist of 10 elements from Scandium (Atomic Number = 21) to Zinc (Atomic Number = 30).
• Second Transition Series or 4d series (5th period)- This series involves the filling of 4d sub-shell and consist of 10 elements from Ytterium (Atomic Number = 39) to Cadmium (Atomic Number = 48).
• Third Transition Series or 5d series (6th period)- This series involves the filling of 5d sub-shell and consist of 10 elements i.e. Lanthanum (Atomic Number = 57) and then from Hafnium (Atomic Number = 72) to Mercury (Atomic Number = 80).
• Fourth Transition Series or 6d series (7th period)- This series involves the filling of 6d sub-shell and consist of 10 elements i.e. Actinium (Atomic Number = 89) and then from Rutherfordium (Atomic Number = 104) to Ununbium (Atomic Number = 112).

Electronic Configuration of Transition Elements:

The general outer electronic configuration of Transition elements is [Noble Gas] (n-1) d^1-10 ns^0-2, where ‘n’ represents an outermost shell. This shows that transition elements have either one or two electrons in the outermost shell i.e. ns except Palladium (Pd) which has no electron in ns orbital while their penultimate shell has (n-1) d^1-10 configuration. The electronic configuration of various series of Transition elements is as under-

Electronic Configuration of 3d Series:

The exceptional electronic configuration of Cr and Cu in this series is due to the fact that these configurations have exactly half-filled and completely filled orbitals which provide them extra-stability due to symmetry and exchange-energy effect.

Electronic Configuration of 4d Series:

Electronic Configuration of 5d Series:

The irregularities in the electronic configuration of elements like Nb, Mo, Ru, Rh, Pd, Pt and Au is due to the change in the relative energies of various sub-shells with a change in atomic number which can be explained by considering factors like-

• Electron-electron interactions.
• Nuclear-electron interactions.
• Exchange energy forces.
• Shielding of one electron by the other electron.

Electronic Configuration of 6d Series:

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