The transition elements are those elements which have incompletely filled (partly filled) d-subshells in their ground state or in any one of their oxidation states.
1) The d-block elements are called transition elements. This block consists of elements lying between s- and p-blocks i.e. , between groups 2 and 13, starting from fourth period.
2) In these elements, the outermost shell contains one or two electrons in their s-orbital (ns) but the last electron enters the last but one d-subshell i.e. , (n – 1) d.
3) The elements of this block have general characteristic properties intermediate between the elements of s-block and p-block, The study of transition metals is important because the precious metals such as silver, gold and platinum and industrially important metals like iron, zinc, palladium, copper, titanium, nickel, chromium, etc. are transition elements.
4) The elements in which the last electron enters the f-orbital of the atoms are called f-block elements. In these elements, the last electron is added to the third to the outermost (called antepenultimate) energy shell i.e., (n-2) . Therefore, these elements are also called inner transition elements.
5) These consist of two rows of elements placed at the bottom of the periodic table and are called lanthanides and actinides. These two series are generated by the filling of characteristic electrons in the 4f (lanthanides) and 5f (actinides) orbitals.
Position of Transition Elements
Coinage metals, Cu, Ag and Au (of group 11) are transition metals because in their commonly occurring oxidation states, they have partly filled d-subshells.
For example: Cu2+ has 3d9 configuration, Ag2+ has 4d9 configuration, Au3+ has 5d8 configuration, although all these three atoms have completely filled d-subshell in their elementary states (Cu : 3d10; Ag : 4d10; Au : 5d10).
Elements of group 12 i.e. , Zn, Cd and Hg, do not have partly filled d-subshells in their elementary state or in their commonly occurring oxidation states. However, being the end members of three transition series these may be treated along with transition elements. They are quite similar to other transition elements in some of their chemical properties.
The d-block or transition elements occupy position in between s- and p-block in groups 3-12 in the periodic table. Starting from fourth period, d-block elements consist of three complete series known as
a) first transition series or 3d series (Sc to Zn)
b) second transition series or 4d series (Y to Cd)
c) third transition series or 5d series (La, Hf to Hg)
d) The fourth transition series or 6d series (beginning with Ac) is still incomplete.
The very name ‘transition’ is given to these elements of d-block because of their position between s- and p-block elements, They represent change (or transition) in properties from most electropositive s-block elements to least electropositive (or most electronegative) p-block elements. In their atoms the d-orbitals of the penultimate (n-1) energy level receive electrons.
Electronic Configuration of Transition Elements
where (n-1) stands for inner shell and d- orbital may have one to ten electrons and the s-orbital of the outermost shell may have one or two electrons.The Transition series consist of three complete rows of ten elements and one incomplete row. These row are called first , second and third transition series which involve the filling of 3d , 4d and 5d orbitals. The fourth row of 6d sub shell is incomplete. These series are also called transition series.
First transition series
The first transition series consists of elements from scandium, Sc (Z =21)to zinc, Zn(Z = 30). In scandium, the 3d-orbital starts filling up and its electronic configuration is [Ar] 4s23d1. As we move from scandium onwards, 3d-orbital get filled up more and more till the last element, zinc, in which the 3d-orbital are completely filled i.e., [Ar]
The configurations of chromium and copper are anomalous
Half-filled and completely-filled electronic configurations (i.e. d5 and d10) have extra stability associated with them. Moreover the energy difference between 3d and 4s-orbitals is not large enough to prevent the electron entering the 3d orbitals. Thus, to acquire increased stability, one of the 4s-electrons goes to nearby 3d-orbitals so that 3d-orbitals become half- filled in case of chromium and completely-filled in case of copper respectively. Therefore, the electronic configuration of Cr is [Ar] 3d54s1 rather than [Ar] 3d4 4s2 while that of Cu is [Ar] 3d10 4s1 instead of [Ar] 3d9 4s2.
Second Transition Series
The second transition series consists of elements from yttrium, Y (Z = 39) to cadmium, Cd (Z = 48), i.e. yttrium (Z = 39), zirconium (Z = 40), niobium (Z = 41), molybdenum (Z = 42), technetium (Z = 43), ruthenium (Z = 44), rhodium (Z = 45), palladium (Z = 46), silver (Z = 47) and cadmium (Z = 48). This series involves the filling of 4d-orbitals.
Third transition series
This series consists of elements lanthanum (Z = 57) and from hafnium (Z = 72) to mercury (Z = 80) i.e ., lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au) and mercury (Hg).
In between lanthanum and hafnium there are fourteen elements called lanthanoids which involve the filling of 4f-orbitals and do not belong to this series.
The elements of this series involve the gradual filling of 5d-orbitals.
In the second and third transition series, there are many anomalous configurations in comparison to those of first transition series. These are attributed to factors like nuclear-electron and electron-electron forces.
Fourth Transition Series
It involves the filling of 6d-subshell starting from actinium (Z = 89); which has the configuration 6d17s2.
In all the transition metals , the ns orbitals are filled before the (n-1) orbitals.
For example : In Scandium , the last electron goes to 4s orbital rather than 3d- orbital because the 4s -orbital lies a little lower than the 3d-orbital.
As the nuclear charge increases, the energies of the subshells decrease.
Up to argon (Z = 18), the expected sequence of 1s, 2s, 2p, 3s and 3p is observed. After filling 3s and 3p-subshells we expect that the next electron in potassium (Z = 19) should enter the 3d-subshell. But, at K (Z = 19) and Ca (Z = 20), 4s-orbital has lower energy due to its greater penetration tendency than 3d-orbital. Therefore, in K and Ca, the outermost electron enters the 4s-orbital. However, after calcium, there is a sharp decrease in the energy of 3d-subshell and its energy becomes less than the 4s-subshell. Therefore, the next electron after calcium goes to 3d-orbital. As the nuclear charge increases from Ca (20) to Zn (30), the 3d-orbitals get filled and become more effective in shielding the 4s-electrons from the nucleus.
After calcium, the energy of 4s-subshell is more than 3d-subshell. Therefore, when electrons are to be removed from these atoms, the electrons will go from 4s-orbital rather than 3d-orbital, though the former was filled earlier.
For example :
Ni = : [Ar] 3d8
The general characteristics of d-block elements
1) Nearly all the transition elements have typical metallic properties such have high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre.
2) Except mercury which is liquid at room temperature, other transition elements have typical metallic structures.
3) They have high melting and boiling points and have higher heats of vaporisation than non-transition elements.
4) The transition elements have very high densities as compared to the metals of groups I and II (s-block).
5) The first ionisation energies of d-block elements are higher than those of s-block elements but are lesser than those of p-block elements.
6) They are electropositive in nature and form coloured compounds.
7) They have good tendency to form complexes.
8) They exhibit several oxidation states and form alloys with other metals.
9) Their compounds are generally paramagnetic in nature.
10) They form interstitial compounds with elements such as hydrogen , boron, carbon , nitrogen etc.
11) Most of the transition metals such as Mn , Ni , Co , Cr , V , Pt and their compounds have been used as good catalyst.