Electronic Configuration

The valence shell configuration of transition elements is (//—l)r/^1-10//^¹², where n indicates the outermost shell and (»-l) indicates the penultimate shell. The f/-block elements are classified in 3d, Ad, 5d and 6d series.

In 3d series (first transition series), it contains 10 elements from Sc_n to Zn₃₀ where the last electron fills in 3d orbital and these elements are kept in 4^th period. After filling two electrons in As orbital of Ca i.e. Is², 2s²2p⁶, 3s²3p⁶As² (4s drops in energy in comparison to 3d), the electrons start filling all the 10 electrons in 3d orbital. During the filling of electrons in 3d, it is observed that half filled and completely filled 3d orbitals have extra stability owing to the lowering of energy due to exchange energy in half filled configuration and neutralisation of spins, lowering the energy in completely filled configuration. This fact comes into light in the configuration of Cr-,₄ and Cu₂₉ in particular. The general configuration of elements is formalised by (Ar)3c/^1_104s^1,2. In case of Cr, so as to gain extra stability, an electron shifts from 4s to 3d sub-shell to acquire half filled configuration i.e. (Ar) 3«^,54.s¹. Similarly, in the configuration of Cu, the completely filled arrangement of electrons is observed. This result in Cu acquiring (Ar) 3r/¹⁰4.s¹ configuration.

Owing to the interplay of the following forces, it is difficult to predict correct configuration of elements in the second, third, and fourth transition series.

(/) Electron-electron repulsion (ii) Nuclear-electron attraction (///) Shielding outermost electron by inner electrons (/v) Exchange forces.

Second Transition Series (4d-series)

The general configuration of these elements is governed by (Kr) 4r/¹⁴⁰5.s⁰“². The series starts from Y₃₉ and ends at the element Cd₄₈. There occur many irregularities in the configuration of these elements eg., Nb₄₁ should have the configuration (Kr)4c/³5s², but actually it has (Kr) 4г/⁴5.$^л; Ru₄₄ should have (Kr) 4c/⁶5s², but it has (Kr) 4d¹5s¹: Rh₄₅ should have (Kr) 4d⁷5s², but it has (Kr) 4d®5s Similarly, Palladium (Pd₄₆) should have (Kr) 3d^&4s², but in actual, it has (Kr) 3c/¹⁰4s°. The cause for this may be attributed to four factors mentioned earlier.

Third Transition Series or 5d-series

The general configuration for these elements is given by (Xe 4f°^{or 14} 5c/^1-10 6s¹-². The series starts from La₅₇ to Elg₈₀ where the electrons fill 5d orbitals Some irregularities in the electronic configuration are observed e.g.. Tungsten (W₇₄) should have the elctronic configuraton as (Xe) 4f^u5d^s6s^l, but actually it has (Xe) 4f¹⁴5d⁶s²: also, Pt₇₈ should have the electronic configuration (Xe) 4/¹⁴5r/^s6.s-², but in reality it has (Xe)4/¹⁴5c/⁹6.s^!.

Fourth Transition Series or 6d-series

This series is incomplete. It does not contain all the 10 elements because all the elements have not been discovered so far, but the place has been left vacant in the periodic table. The names and symbols of some of the elements known till date are not in agreement among the chemists. These elements have radioactivity property, but have short half-life and their chemistry has not been investigated so far.

Electronic Configuration of Transition Metal Ions

Metals can only lose electrons to form cations. In transition elements, electrons can be lost from the outer most and penultimate shells. At the time of filling, 4s orbital was filled first as there was drop in the energy of 45 and then comes the turn of 3d orbital. This pattern holds for filling of electrons. When it comes to the loss the electrons, the orbtials 45 and 3d, readjust their energies and 45 becomes outermost and 3d becomes penultimate. As a result, the electrons are first lost from 4s and then from 3d orbtials.

For example:

It is noticed that electrons are not lost from 3d in case d-orbital is fully filled.

For example:

Electronic configuration of atoms and ions belonging to first transition series are given in Table:

Some important physical constants of transition elements have been listed in Table for 3d, Table for Ad and Table for 5d, for the first, second and third transition series for reference. The physical properties compared are metallic behaviour, melting point, varying oxidation states, paramagnetism and colour of transition metal ions. By comparison, .s-block elements are also metals, but they are soft with low melting points and show low oxidation states and are diamagnetic. Within transition series, the elements have similar, but graded properties and they differ in properties with s and p-block elements.