Transition elements are the metals that shape our world, from iron and copper to gold and platinum. This article explains their position in the periodic table, classification, electronic configurations and key characteristics in a simple, structured way. The concepts here apply equally to school chemistry, competitive examinations and higher studies in inorganic and coordination chemistry.
Transition Elements or d-Block Elements and Their Positions in the Periodic Table
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.
This definition covers coinage metals, Cu, Ag and Au (of group 11) as 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). However, it may be noted that the above definition does not include elements of group 12 (previously called II B) i.e., Zn, Cd and Hg. These elements 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 elements which lie in-between s and p block elements in the long form of periodic table (i.e., elements of groups 3–12) are called transition elements.
The 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. Further, as they are metallic in nature, the name transition metals is often used for these elements. In their atoms, the d-orbitals of the penultimate (n – 1) energy level receive electrons.
Electronic Configurations of Transition Elements
In the transition elements, the d-orbitals are successively filled. These elements are called d-block elements since in them, 3d, 4d, 5d and 6d subshells are incomplete and the last electron enters the (n – 1)d orbital, i.e., penultimate (last but one) shell. Their general electronic configuration is :
(n – 1)d¹⁻¹⁰ ns¹⁻² where n is the outermost shell
where (n – 1) stands for inner shell (or penultimate shell) and d-orbitals may have one to ten electrons and the s-orbital of the outermost shell (n) may have one or two electrons.
Classification of d-Block Elements
Unlike s or p block elements which are usually discussed as columns or groups, d-block elements are better discussed by classifying them into horizontal series. In the periodic table, there are four main transition series of elements corresponding to filling of 3d, 4d, 5d and 6d sublevels in 4th, 5th, 6th and 7th periods.
(i) First transition series or 3d series corresponding to filling of 3d sublevel consists of the following 10 elements of 4th period : Sc (At. No. = 21) Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn (At. No. = 30)
(ii) Second transition series or 4d series corresponding to filling of 4d sublevel consists of the following 10 elements of 5th period : Y (At. No. = 39) Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag and Cd (At. No. = 48)
(iii) Third transition series or 5d series corresponding to filling of 5d sublevel consists of the following 10 elements of 6th period.
La (At. No. = 57) ; Hf (At. No. = 72), Ta, W, Re, Os, Ir, Pt, Au and Hg (At. No. = 80)
(iv) Fourth transition series or 6d series corresponding to the filling of 6d sublevel starts with Actinium (At. No. = 89) followed by elements with atomic number 104 onwards.
Let us discuss the electronic configurations of individual members of these
series.
First Transition Series of d-Block Elements
The first transition series consists of elements from scandium, Sc (Z = 21) to zinc, Zn (Z = 30) i.e., scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. In scandium, the 3d-orbital starts filling up and its electronic configuration is [Ar] 4s²3d¹. As we move from scandium onwards, 3d-orbitals get filled up more and more till the last element, zinc, in which the 3d-orbitals are completely filled, i.e., [Ar] 4s²3d¹⁰.
| Element | Symbol | Atomic number | Electronic configuration |
| Scandium | Sc | 21 | [Ar] 3d¹4s² |
| Titanium | Ti | 22 | [Ar] 3d²4s² |
| Vanadium | V | 23 | [Ar] 3d³4s² |
| Chromium | Cr | 24 | [Ar] 3d⁵4s¹ |
| Manganese | Mn | 25 | [Ar] 3d⁵4s² |
| Iron | Fe | 26 | [Ar] 3d⁶4s² |
| Cobalt | Co | 27 | [Ar] 3d⁷4s² |
| Nickel | Ni | 28 | [Ar] 3d⁸4s² |
| Copper | Cu | 29 | [Ar] 3d¹⁰4s¹ |
| Zinc | Zn | 30 | [Ar] 3d¹⁰4s² |
Exceptional Electronic Configurations of Cr and Cu
It may be noted that the configurations of chromium and copper are anomalous. We know that 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. Therefore, the electronic configuration of Cr is [Ar] 3d⁵4s¹ rather than [Ar] 3d⁴4s² while that of Cu is [Ar] 3d¹⁰4s¹ instead of [Ar] 3d⁹4s².
Thus, the electronic configuration of chromium (Z = 24) and copper (Z = 29) are 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ and 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ respectively.
Second Transition Series of d-Block Elements
The second transition series consists of elements from yttrium, Y (Z = 39) to cadmium, Cd (Z = 48), i.e., yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver and cadmium. This series involves the filling of 4d-orbitals.
| Element | Symbol | Atomic number | Electronic configuration |
| Yttrium | Y | 39 | [Kr] 4d¹5s² |
| Zirconium | Zr | 40 | [Kr] 4d²5s² |
| Niobium | Nb | 41 | [Kr] 4d⁴5s¹ |
| Molybdenum | Mo | 42 | [Kr] 4d⁵5s¹ |
| Technetium | Tc | 43 | [Kr] 4d⁶5s¹ |
| Ruthenium | Ru | 44 | [Kr] 4d⁷5s¹ |
| Rhodium | Rh | 45 | [Kr] 4d⁸5s¹ |
| Palladium | Pd | 46 | [Kr] 4d¹⁰5s⁰ |
| Silver | Ag | 47 | [Kr] 4d¹⁰5s¹ |
| Cadmium | Cd | 48 | [Kr] 4d¹⁰5s² |
| Noteworthy Point |
|---|
| The element technetium (Tc) is a synthetic transition metal which was made artificially. This was so named, because the word technetium means artificial. |
| Pd has no electron in the ns-subshell. Pd (Z = 46) : [Kr] 4d¹⁰5s⁰ |
Third Transition Series of d-Block Elements
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.
| Element | Symbol | Atomic number | Electronic configuration |
| Lanthanum | La | 57 | [Xe] 4f⁰5d¹6s² |
| Hafnium | Hf | 72 | [Xe] 4f¹⁴5d²6s² |
| Tantalum | Ta | 73 | [Xe] 4f¹⁴5d³6s² |
| Tungsten | W | 74 | [Xe] 4f¹⁴5d⁴6s² |
| Rhenium | Re | 75 | [Xe] 4f¹⁴5d⁵6s² |
| Osmium | Os | 76 | [Xe] 4f¹⁴5d⁶6s² |
| Iridium | Ir | 77 | [Xe] 4f¹⁴5d⁷6s² |
| Platinum | Pt | 78 | [Xe] 4f¹⁴5d⁹6s¹ |
| Gold | Au | 79 | [Xe] 4f¹⁴5d¹⁰6s¹ |
| Mercury | Hg | 80 | [Xe] 4f¹⁴5d¹⁰6s² |
| Noteworthy Point |
|---|
| In La (Z = 57) one electron goes to 5d-orbital before filling of 4f-orbital (an exception from Aufbau order). |
It may be noted that 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 of d-Block Elements
It involves the filling of 6d-subshell starting from actinium (Z = 89); which has the configuration 6d¹7s². It includes elements with atomic number 104 onwards as given below :
| Element | Symbol | Atomic number | Electronic configuration |
| Actinium | Ac | 89 | [Rn] 5f⁰6d¹7s² |
| Rutherfordium | Rf | 104 | [Rn] 5f¹⁴6d²7s² |
| Hafnium | Ha | 105 | [Rn] 5f¹⁴6d³7s² |
| Seaborgium | Sg | 106 | [Rn] 5f¹⁴6d⁴7s² |
| Bohrium | Bh | 107 | [Rn] 5f¹⁴6d⁵7s² |
| Hassium | Hs | 108 | [Rn] 5f¹⁴6d⁶7s² |
| Meitnerium | Mt | 109 | [Rn] 5f¹⁴6d⁷7s² |
| Darmstadtium | Ds | 110 | [Rn] 5f¹⁴6d⁸7s² |
| Roentgenium | Rg | 111 | [Rn] 5f¹⁴6d¹⁰7s¹ |
| Copernicium | Cn | 112 | [Rn] 5f¹⁴6d¹⁰7s² |
**Note : It may be noted that recently, the IUPAC has approved the name copernicium with symbol Cn for the element with atomic number 112.
We have learnt that in all the transition metals, the ns-orbitals are filled before the (n – 1)d-orbitals. For example, in scandium (Z = 21), the last electron goes to 4s-orbital rather than 3d-orbital because the 4s-orbital lies a little lower than the 3d-orbital. However, it has been observed that a reversal of the relative order of 3d– and 4s-orbitals occurs when the 3d-orbitals have been fed with some electrons. The change in energy levels of the various orbitals can be easily understood by considering the effect of nuclear charge. The variation in relative energies of various subshells with the change in atomic number is shown in Figure. It is observed that as the nuclear charge increases, the energies of the subshells decrease. Upto 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 (shown in a circle in Figure). 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.
| Noteworthy Point |
|---|
| The fundamental difference in the elec tronic configurations of transition elements and representative elements is that in the representative elements (belonging to s– and p-block), the valence electrons are present only in the outermost shell. On the other hand, in the transition elements, the valence electrons are present in the outermost shell (ns) as well as d-orbitals of penultimate (n-1) shell. |
Ionisation of Transition Elements or d-Block Elements
We have observed that 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,
Fe (Z = 26) : [Ar] 3d⁶4s²
Fe²⁺ [Ar] 3d⁶ and not [Ar] 4s²3d⁴
Ni (Z = 28) : [Ar] 3d⁸4s²
Ni²⁺ : [Ar] 3d⁸ and not [Ar] 4s²3d⁶
The same trend is repeated for 5s, 4d and 6s, 5d-orbitals in second and third transition series. In the lanthanum, the energies of 4f, 5d and 6s-orbitals lie very close to one another and one electron goes to 5d-orbital before 4f-orbitals. Its configuration is 5d¹6s². In the other elements, 4f-orbitals are stabilized relative to 5d and electrons go into 4f-orbitals.
Definition of Transition Elements or d-Block Elements
Strictly speaking, transition elements are those elements which have partly filled (n – 1)d subshell in their elementary state.
Let us examine the electronic configuration of copper atom, cuprous ion and cupric ion.
Cu (Z = 29) = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹
Cu⁺ (Cuprous ion) = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰
Cu²⁺ (Cupric ion) = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹
Obviously, copper in the ground state or as cuprous ion cannot be considered as transition element and rightly so it does not exhibit the characteristic properties of the transition elements. However, cupric ion has incomplete d-sublevel, i.e., d⁹ electronic configuration and hence is a transition metal ion.
A transition element may thus be defined as the element whose atom in ground state or ion in one of the common oxidation states, has incomplete d-subshell (partly filled), i.e., having electrons between 1 to 9.
This definition excludes zinc, cadmium and mercury from the transition elements because they do not have partly filled (or incomplete) d-subshell in their atomic state or their common oxidation state (i.e., Zn²⁺, Cd²⁺, Hg²⁺). The d-subshell configurations of these atoms or their dipositive ions are 3d¹⁰, 4d¹⁰ and 5d¹⁰. They do not show properties of transition elements to any appreciable extent except for their ability to form complexes. However, being the end members of the three transition series, they are generally studied with the d-block elements.
General Characteristics of d-block Elements or Transition Elements
The general characteristics of d-block elements are :
(1) Except for mercury which is a liquid, all transition elements have typical metallic structure and show typical metallic properties such as conductivity, malleability and ductility, lustre, high tensile strength etc.
(2) Their atomic radii are in-between those of s– and p-block elements. In a series, they decrease with increase in atomic number but the decrease is small after midway.
(3) They have high melting and boiling points, high enthalpies of vaporisation, high enthalpies of atomisation and high enthalpies of hydration of their ions. These properties depend upon the strength of the metallic bond in them.
(4) Their first ionization energies are higher than those of s-block elements and less than those of p-block elements.
(5) They are electropositive in nature.
(6) They show variable oxidation states.
(7) The stability of any oxidation state or the tendency for a transition metal ion to act as oxidizing or reducing agent depends upon its electrode potential.
(8) A number of these transition metals and their compounds show catalytic properties.
(9) Most of the transition elements form coloured compounds.
(10) Their compounds are generally paramagnetic in nature.
(11) They have a great tendency to form complexes.
(12) They form interstitial compounds with elements like H, C, B and N.
(13) They form alloys.
Exceptional Behaviour of Zinc, Cadmium and Mercury
As already mentioned, due to completely filled d¹⁰ configuration, Zn, Cd and Hg show exceptional behaviour as compared to the other members of their respective series.
How does zinc differ from other elements of the first transition series?
Taking the example of first transition series, zinc differs from the other members in the following characteristics :
(i) It has much higher atomic radius and ionization enthalpies.
(ii) Its compounds are white while those of the other members are coloured.
(iii) It does not show variable oxidation states unlike other members.
(iv) Its compounds (containing Zn²⁺ ions) are diamagnetic while those of the other members are paramagnetic.
(v) It forms lesser number of complexes than other members.
Important Questions and Answers
What are transition elements and Why are d-block elements called transition elements ?
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. The name ‘transition’ is given to these elements because of their position between s-block and p-block elements. They represent a change in properties from the most electropositive s-block elements to the least electropositive p-block elements.
Where are d-block elements located in the periodic table and What is the general electronic configuration of transition elements?
The elements which lie between s-block and p-block elements in the long form of the periodic table, i.e., elements of Groups 3–12, are called d-block elements. The general electronic configuration of transition elements is :
(n − 1)d¹⁻¹⁰ ns¹⁻²
Here, (n − 1) represents the penultimate shell, and n represents the outermost shell.
How many transition series are there? Which elements constitute the first, second, third and fourth transition series?
There are four main transition series corresponding to the filling of 3d, 4d, 5d and 6d subshells in the 4th, 5th, 6th and 7th periods, respectively.
The first transition series consists of ten elements from scandium (Sc, atomic number 21) to zinc (Zn, atomic number 30) : Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.
The second transition series consists of ten elements from yttrium (Y, atomic number 39) to cadmium (Cd, atomic number 48) : Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag and Cd.
The third transition series includes lanthanum (La, atomic number 57) and the elements from hafnium (Hf, atomic number 72) to mercury (Hg, atomic number 80).
The fourth transition series involves the filling of the 6d-subshell. It starts with actinium (Ac, atomic number 89) and includes elements with atomic numbers 104 onwards.
Why do chromium and copper have exceptional electronic configurations?
Half-filled and completely filled d-subshells have extra stability. Therefore, one 4s electron shifts to the 3d-subshell in chromium and copper.
Chromium (Cr, Z = 24) : [Ar] 3d⁵ 4s¹
Copper (Cu, Z = 29) : [Ar] 3d¹⁰ 4s¹
Why is technetium noteworthy among transition elements?
Technetium (Tc) is a synthetic transition metal produced artificially. Its name means artificial.
What is the electronic configuration of palladium?
Palladium (Pd, Z = 46) has the electronic configuration [Kr] 4d¹⁰ 5s⁰. It has no electron in the 5s-subshell.
Why are 4s electrons removed before 3d electrons during ionisation?
After calcium, the relative energies of the 3d– and 4s-orbitals change. The 4s electrons are removed before the 3d electrons during ionisation. For example :
Fe (Z = 26) : [Ar] 3d⁶ 4s²
Fe²⁺ : [Ar] 3d⁶
Ni (Z = 28) : [Ar] 3d⁸ 4s²
Ni²⁺ : [Ar] 3d⁸
Why are zinc, cadmium and mercury generally excluded from transition elements?
Zinc, cadmium and mercury have completely filled d-subshells in their atomic states and common dipositive ions.
Zn and Zn²⁺ : 3d¹⁰
Cd and Cd²⁺ : 4d¹⁰
Hg and Hg²⁺ : 5d¹⁰
However, they are generally studied along with d-block elements because they are the end members of the three transition series and show similarities in some chemical properties.
What is the fundamental difference between the electronic configurations of transition elements and representative elements?
In representative elements belonging to the s-block and p-block, valence electrons are present only in the outermost shell. In transition elements, valence electrons are present in both the outermost ns-orbitals and the penultimate (n − 1)d-orbitals.
Why are the electronic configurations of some second and third transition-series elements anomalous?
Many elements of the second and third transition series show anomalous electronic configurations due to factors such as nuclear–electron and electron–electron interactions.