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Chapter 4 The D– And F– Block Elements
Position In The Periodic Table
The d-block elements, comprising Groups 3-12, are located in the middle section of the periodic table. They are characterized by the filling of d orbitals in the penultimate energy level across four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg), and 6d (Ac, Rf to Cn). The f-block elements (lanthanoids and actinoids) are placed separately at the bottom, with the filling of 4f and 5f orbitals, respectively. Transition metals are defined as elements with incompletely filled d subshells in their neutral atom or common ions. Elements like Zn, Cd, and Hg, with completely filled d¹⁰ configurations, are generally excluded from this definition despite being at the end of transition series.
Electronic Configurations Of The D-Block Elements
The general electronic configuration of d-block elements is $(n-1)d^{1-10}ns^{1-2}$. However, exceptions exist due to the small energy difference between $(n-1)d$ and $ns$ orbitals and the stability of half-filled ($d^5$) and completely filled ($d^{10}$) subshells. For example, Chromium (Cr) has $3d^5 4s^1$ instead of $3d^4 4s^2$, and Copper (Cu) has $3d^{10} 4s^1$ instead of $3d^9 4s^2$. The elements of Group 12 (Zn, Cd, Hg) have the configuration $(n-1)d^{10}ns^2$ and are not considered transition metals because their d subshells are completely filled.
General Properties Of The Transition Elements (D-Block)
Transition elements exhibit characteristic properties due to their incompletely filled d orbitals.
Physical Properties
They display typical metallic properties: high tensile strength, ductility, malleability, high thermal and electrical conductivity, and metallic lustre. Most have high melting and boiling points and high enthalpies of atomization, attributed to the involvement of $(n-1)d$ electrons in metallic bonding. Melting points generally increase across a series to a maximum near the middle (d⁵ configuration) and then decrease.
Variation In Atomic And Ionic Sizes Of Transition Metals
Atomic and ionic radii generally decrease across a transition series due to increasing nuclear charge, with less effective shielding by d electrons. However, the decrease is less pronounced than in s- and p-block elements. Notably, the atomic radii of the second (4d) and third (5d) transition series are very similar due to the lanthanoid contraction, which compensates for the expected increase in size with higher atomic number.
Ionisation Enthalpies
Ionization enthalpies generally increase across a transition series due to increasing nuclear charge. The variation is less steep than in non-transition elements. Successive ionization enthalpies also show trends related to achieving stable electron configurations (e.g., $d^5$ in Mn²⁺ and $d^{10}$ in Zn²⁺).
Oxidation States
Transition metals exhibit variable oxidation states due to the involvement of both $ns$ and $(n-1)d$ electrons in bonding. The most common oxidation state is +2. Early members show lower oxidation states, while those in the middle (like Mn) show a wider range, including high oxidation states stabilized by electronegative elements like oxygen and fluorine. The stability of higher oxidation states generally decreases after manganese.
Trends In The M2+/M Standard Electrode Potentials
The standard electrode potentials ($E^o$) for the $M^{2+}/M$ couple generally increase (become less negative) across the 3d series, reflecting a general trend of decreasing reactivity. However, there are irregularities, with more negative values for Mn, Ni, and Zn due to factors like the stability of $d^5$ (Mn²⁺) and $d^{10}$ (Zn²⁺) configurations, and high enthalpy of hydration for Ni²⁺.
Trends In The M3+/M2+ Standard Electrode Potentials
The $E^o$ values for $M^{3+}/M^{2+}$ couples also show variations. High stability of $d^5$ configuration makes $Mn^{2+}$ resistant to oxidation, leading to a positive $E^o$ for $Mn^{3+}/Mn^{2+}$. Conversely, $Fe^{3+}$ ($d^5$) is more stable than $Fe^{2+}$ ($d^6$), resulting in a positive $E^o$ for $Fe^{3+}/Fe^{2+}$.
Trends In Stability Of Higher Oxidation States
The stability of higher oxidation states is generally enhanced by electronegative elements like oxygen and fluorine. Oxides often stabilize higher oxidation states than fluorides (e.g., Mn₂O₇ vs. MnF₄). Oxoanions also stabilize high oxidation states.
Chemical Reactivity And Eo Values
Most transition metals are reactive and dissolve in mineral acids, except for noble metals like Cu (reacts only with oxidizing acids) and noble metals further down the group. The $E^o$ values indicate that metals with more negative potentials are stronger reducing agents and more reactive.
Magnetic Properties
Transition metal ions exhibit paramagnetism due to the presence of unpaired d-electrons. The magnetic moment ($μ$) is calculated using the spin-only formula $μ = \sqrt{n(n+2)}$ BM, where $n$ is the number of unpaired electrons. Magnetic moment generally increases with $n$. Ions with $d^0$ or $d^{10}$ configurations are typically diamagnetic.
Formation Of Coloured Ions
Most transition metal ions in aqueous solutions are coloured. This is because the d orbitals are split into different energy levels in the presence of ligands (crystal field theory). Absorption of light corresponding to certain frequencies causes excitation of d electrons to higher energy levels. The complementary color of the absorbed light is what we observe. The specific color depends on the metal ion and the ligand.
Formation Of Complex Compounds
Transition metals readily form complex compounds due to their small ionic sizes, high ionic charges, and the availability of empty d orbitals for coordination with ligands (anions or neutral molecules).
Catalytic Properties
Transition metals and their compounds are excellent catalysts due to their ability to adopt variable oxidation states and form intermediate complexes with reactants. This facilitates alternative reaction pathways with lower activation energies. Examples include V₂O₅ in the Contact process and finely divided Fe in the Haber process.
Formation Of Interstitial Compounds
Transition metals form interstitial compounds when small atoms (like H, C, N) occupy the spaces (interstices) within their crystal lattices. These compounds are often non-stoichiometric, hard, have high melting points, retain metallic conductivity, and are chemically inert.
Alloy Formation
Transition metals readily form alloys with each other and with other metals due to their similar atomic sizes and metallic properties. These alloys often exhibit enhanced hardness and higher melting points compared to the pure metals.
Some Important Compounds Of Transition Elements
Oxides And Oxoanions Of Metals
Transition metals form oxides with various oxidation states, generally becoming more acidic with increasing oxidation number. For example, $Mn_2O_7$ (Mn in +7 state) is acidic, while $V_2O_5$ is amphoteric, and $CrO$ is basic. Oxoanions like chromate ($CrO_4^{2-}$) and permanganate ($MnO_4^-$) are important compounds with strong oxidizing properties. Potassium dichromate ($K_2Cr_2O_7$) is prepared from chromite ore and is a strong oxidizing agent in acidic media (Cr(VI) → Cr(III)). Potassium permanganate ($KMnO_4$) is prepared from $MnO_2$ and is a powerful oxidizing agent in acidic, basic, or neutral media, with its redox potential depending on the pH.
The Lanthanoids
The lanthanoids are a series of 14 elements following lanthanum (La, Z=57), characterized by the filling of the 4f orbitals. Their general electronic configuration is $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. The most common oxidation state is +3, due to the stability of the $4f^0$, $4f^7$, or $4f^{14}$ configurations.
Electronic Configurations
General configuration is $[Xe] 4f^n 6s^2$, with some exceptions where a 5d electron is present ($Ce, Gd, Lu$). Tripositive ions ($Ln^{3+}$) typically have $4f^n$ configurations.
Atomic And Ionic Sizes
Atomic and ionic radii show a gradual decrease from La³⁺ to Lu³⁺, known as lanthanoid contraction. This is due to the imperfect shielding of the increasing nuclear charge by the simultaneously filling 4f electrons. This contraction leads to lanthanoids having chemical properties similar to each other and affects the properties of elements in the subsequent transition series (e.g., Zr and Hf have similar radii).
Oxidation States
The most common oxidation state is +3. However, +2 and +4 states are also observed, particularly when they lead to stable empty ($4f^0$), half-filled ($4f^7$), or fully-filled ($4f^{14}$) f subshells (e.g., Ce⁴⁺, Eu²⁺, Yb²⁺, Tb⁴⁺).
General Characteristics
Lanthanoids are silvery-white, soft metals with high melting points (except Sm) and densities. They are good conductors of heat and electricity. They react with water and acids, form oxides ($Ln_2O_3$) and hydroxides ($Ln(OH)_3$) that are basic, and their tripositive ions are often coloured and paramagnetic due to unpaired f electrons. Their first ionization enthalpies are comparable to those of alkaline earth metals.
Applications: Used in alloys (mischmetall for lighter flints), catalysts (oxides in petroleum cracking), and phosphors in screens.
The Actinoids
The actinoids comprise 14 elements following actinium (Ac, Z=89), characterized by the filling of 5f orbitals. Their general electronic configuration is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. They are all radioactive, with heavier elements having very short half-lives. Their chemistry is more complex than lanthanoids due to the greater involvement of 5f electrons in bonding and a wider range of oxidation states.
Electronic Configurations
General configuration is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. Unlike lanthanoids, the 5f electrons are less effectively shielded and participate more readily in chemical bonding.
Ionic Sizes
Actinoid contraction, similar to lanthanoid contraction, occurs due to poor shielding by 5f electrons, leading to a more pronounced decrease in size across the series.
Oxidation States
Actinoids exhibit a wide range of oxidation states, with +3 being common but higher states (+4, +5, +6, +7) appearing more frequently and being more stable than in lanthanoids, especially in the earlier part of the series (e.g., U, Np, Pu). This variability is attributed to the similar energies of 5f, 6d, and 7s orbitals.
General Characteristics And Comparison With Lanthanoids
Actinoid metals are silvery and highly reactive. Their magnetic properties are more complex. While they share some similarities with lanthanoids, the greater involvement of 5f electrons in bonding leads to significant differences in chemical reactivity and stability of oxidation states, especially in the early members of the series.
Some Applications Of D- And F-Block Elements
Transition metals and their compounds have vital applications:
- Materials: Iron and steel are crucial for construction. Alloys containing transition metals (e.g., stainless steel) have enhanced properties.
- Industry: Many transition metals and compounds act as catalysts (e.g., V₂O₅ for SO₂ oxidation, Fe for Haber process, Ni for hydrogenation, PdCl₂ in Wacker process).
- Other Uses: AgBr in photography, TiO₂ as pigment, MnO₂ in dry batteries, Cu/Ni alloys in coinage. Lanthanoid alloys (mischmetall) are used in lighter flints, and their oxides in phosphors for screens.
Intext Questions
Question 4.1. Silver atom has completely filled d orbitals ($4d^{10}$) in its ground state. How can you say that it is a transition element?
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Question 4.2. In the series Sc (Z = 21) to Zn (Z = 30), the enthalpy of atomisation of zinc is the lowest, i.e., 126 kJ $mol^{–1}$. Why?
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Question 4.3. Which of the 3d series of the transition metals exhibits the largest number of oxidation states and why?
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Question 4.4. The $E^\circ(M^{2+}/M)$ value for copper is positive (+0.34V). What is possible reason for this? (Hint: consider its high $\Delta_aH^\circ$ and low $\Delta_{hyd}H^\circ$)
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Question 4.5. How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?
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Question 4.6. Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only?
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Question 4.7. Which is a stronger reducing agent $Cr^{2+}$ or $Fe^{2+}$ and why ?
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Question 4.8. Calculate the ‘spin only’ magnetic moment of $M^{2+}(aq)$ ion (Z = 27).
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Question 4.9. Explain why Cu+ ion is not stable in aqueous solutions?
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Question 4.10. Actinoid contraction is greater from element to element than lanthanoid contraction. Why?
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Exercises
Question 4.1. Write down the electronic configuration of:
(i) $Cr^{3+}$
(ii) $Pm^{3+}$
(iii) $Cu^+$
(iv) $Ce^{4+}$
(v) $Co^{2+}$
(vi) $Lu^{2+}$
(vii) $Mn^{2+}$
(viii) $Th^{4+}$
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Question 4.2. Why are $Mn^{2+}$ compounds more stable than $Fe^{2+}$ towards oxidation to their +3 state?
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Question 4.3. Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?
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Question 4.4. To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.
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Question 4.5. What may be the stable oxidation state of the transition element with the following d electron configurations in the ground state of their atoms : $3d^3$, $3d^5$, $3d^8$ and $3d^4$?
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Question 4.6. Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.
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Question 4.7. What is lanthanoid contraction? What are the consequences of lanthanoid contraction?
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Question 4.8. What are the characteristics of the transition elements and why are they called transition elements? Which of the d-block elements may not be regarded as the transition elements?
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Question 4.9. In what way is the electronic configuration of the transition elements different from that of the non transition elements?
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Question 4.10. What are the different oxidation states exhibited by the lanthanoids?
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Question 4.11. Explain giving reasons:
(i) Transition metals and many of their compounds show paramagnetic behaviour.
(ii) The enthalpies of atomisation of the transition metals are high.
(iii) The transition metals generally form coloured compounds.
(iv) Transition metals and their many compounds act as good catalyst.
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Question 4.12. What are interstitial compounds? Why are such compounds well known for transition metals?
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Question 4.13. How is the variability in oxidation states of transition metals different from that of the non transition metals? Illustrate with examples.
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Question 4.14. Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
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Question 4.15. Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
(i) iodide
(ii) iron(II) solution and
(iii) $H_2S$
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Question 4.16. Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron(II) ions (ii) $SO_2$ and (iii) oxalic acid? Write the ionic equations for the reactions.
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Question 4.17. For $M^{2+}/M$ and $M^{3+}/M^{2+}$ systems the $E^\ominus$ values for some metals are as follows:
$Cr^{2+}/Cr \quad -0.9V \quad Cr^{3+}/Cr^{2+} \quad -0.4 V$
$Mn^{2+}/Mn \quad -1.2V \quad Mn^{3+}/Mn^{2+} \quad +1.5 V$
$Fe^{2+}/Fe \quad -0.4V \quad Fe^{3+}/Fe^{2+} \quad +0.8 V$
Use this data to comment upon:
(i) the stability of $Fe^{3+}$ in acid solution as compared to that of $Cr^{3+}$ or $Mn^{3+}$ and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
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Question 4.18. Predict which of the following will be coloured in aqueous solution? $Ti^{3+}$, $V^{3+}$, $Cu^+$, $Sc^{3+}$, $Mn^{2+}$, $Fe^{3+}$ and $Co^{2+}$. Give reasons for each.
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Question 4.19. Compare the stability of +2 oxidation state for the elements of the first transition series.
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Question 4.20. Compare the chemistry of actinoids with that of the lanthanoids with special reference to:
(i) electronic configuration
(ii) atomic and ionic sizes and
(iii) oxidation state
(iv) chemical reactivity.
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Question 4.21. How would you account for the following:
(i) Of the $d^4$ species, $Cr^{2+}$ is strongly reducing while manganese(III) is strongly oxidising.
(ii) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.
(iii) The $d^1$ configuration is very unstable in ions.
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Question 4.22. What is meant by ‘disproportionation’? Give two examples of disproportionation reaction in aqueous solution.
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Question 4.23. Which metal in the first series of transition metals exhibits +1 oxidation state most frequently and why?
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Question 4.24. Calculate the number of unpaired electrons in the following gaseous ions: $Mn^{3+}$, $Cr^{3+}$, $V^{3+}$ and $Ti^{3+}$. Which one of these is the most stable in aqueous solution?
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Question 4.25. Give examples and suggest reasons for the following features of the transition metal chemistry:
(i) The lowest oxide of transition metal is basic, the highest is amphoteric/acidic.
(ii) A transition metal exhibits highest oxidation state in oxides and fluorides.
(iii) The highest oxidation state is exhibited in oxoanions of a metal.
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Question 4.26. Indicate the steps in the preparation of:
(i) $K_2Cr_2O_7$ from chromite ore.
(ii) $KMnO_4$ from pyrolusite ore.
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Question 4.27. What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.
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Question 4.28. What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements : 29, 59, 74, 95, 102, 104.
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Question 4.29. The chemistry of the actinoid elements is not so smooth as that of the lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements.
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Question 4.30. Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element.
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Question 4.31. Use Hund’s rule to derive the electronic configuration of $Ce^{3+}$ ion, and calculate its magnetic moment on the basis of ‘spin-only’ formula.
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Question 4.32. Name the members of the lanthanoid series which exhibit +4 oxidation states and those which exhibit +2 oxidation states. Try to correlate this type of behaviour with the electronic configurations of these elements.
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Question 4.33. Compare the chemistry of the actinoids with that of lanthanoids with reference to:
(i) electronic configuration
(ii) oxidation states and
(iii) chemical reactivity.
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Question 4.34. Write the electronic configurations of the elements with the atomic numbers 61, 91, 101, and 109.
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Question 4.35. Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:
(i) electronic configurations
(ii) oxidation states
(iii) ionisation enthalpies and
(iv) atomic sizes.
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Question 4.36. Write down the number of 3d electrons in each of the following ions: $Ti^{2+}$, $V^{2+}$, $Cr^{3+}$, $Mn^{2+}$, $Fe^{2+}$, $Fe^{3+}$, $Co^{2+}$, $Ni^{2+}$ and $Cu^{2+}$. Indicate how would you expect the five 3d orbitals to be occupied for these hydrated ions (octahedral).
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Question 4.37. Comment on the statement that elements of the first transition series possess many properties different from those of heavier transition elements.
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Question 4.38. What can be inferred from the magnetic moment values of the following complex species ?
| Example | Magnetic Moment (BM) |
|---|---|
| $K_4[Mn(CN)_6]$ | 2.2 |
| $[Fe(H_2O)_6]^{2+}$ | 5.3 |
| $K_2[MnCl_4]$ | 5.9 |
Answer: