Chapter 4: The d-and f-Block Elements | Summary

Transition metals (except Zn, Cd, Hg, and Mn) exhibit typical metallic properties: high tensile strength, ductility, malleability, high thermal and electrical conductivity, and metallic lustre. Their melting and boiling points are high due to the involvement of (n–1)d electrons in interatomic metallic bonding, in addition to ns electrons. Melting points in each series rise to a maximum near d5 configuration (except for anomalous Mn and Tc) and then fall.

The enthalpies of atomisation are high and show a maximum near the middle of each series, indicating that one unpaired electron per d orbital is particularly favourable for strong interatomic interaction. The density of these metals generally increases from Ti to Cu due to the decrease in metallic radius coupled with increasing atomic mass.

Within a transition series, ionic and atomic radii show a gradual decrease with increasing atomic number due to poor shielding by d electrons. Comparing across series, atomic radii increase from the 3d to the 4d series but the 5d series has radii virtually identical to those of the 4d series. This is due to the lanthanoid contraction — the steady decrease in atomic and ionic size across the lanthanoid series caused by poor shielding of one 4f electron by another.

The lanthanoid contraction results in Zr (160 pm) and Hf (159 pm) having almost identical radii, making their separation in nature extremely difficult. The second and third d-series elements therefore exhibit very similar physical and chemical properties.

Ionisation enthalpies of transition metals increase from left to right across each series but less steeply than in non-transition elements because d electrons provide partial shielding. The first ionisation enthalpy shows an irregular trend in the 3d series due to changes in relative energies of 4s and 3d orbitals upon electron removal. Extra stability at d5 and d10 configurations causes notable irregularities: high second ionisation enthalpy for Cr and Cu (d5 and d10 as M+) and relatively low value for Zn.

Exchange energy stabilises half-filled and fully filled d configurations, making the third ionisation enthalpy of Fe lower than that of Mn (because Fe3+ has d5 configuration). Similarly, high third ionisation enthalpies of Cu, Ni and Zn explain why these elements rarely exhibit oxidation states above +2.

A characteristic feature of transition metals is the display of variable oxidation states, arising from the availability of both ns and (n–1)d electrons for bond formation. Oxidation states differ by unity (e.g., VIII, VIII, VIV, VV), unlike non-transition elements where they typically differ by two. Elements near the middle of the series (especially Mn, which shows +2 to +7) exhibit the greatest number of oxidation states.

The maximum oxidation state corresponds to the total number of s and d electrons up to Mn. Beyond Mn, higher oxidation states become progressively less stable. Higher oxidation states are stabilised by oxygen and fluorine due to their high electronegativity, small size, and ability to form multiple bonds. Low oxidation states (even zero, as in Ni(CO)4 and Fe(CO)5) are stabilised by π-acceptor ligands.

The Eo(M2+/M) values for the first transition series show a general trend towards less negative values across the series, related to increasing sum of the first and second ionisation enthalpies. Cu has a unique positive Eo (+0.34 V) because its high enthalpy of atomisation and low hydration enthalpy make the transformation Cu(s) → Cu2+(aq) energetically unfavourable, preventing it from dissolving in non-oxidising acids.

For the M3+/M2+ couples, Mn3+ and Co3+ are strong oxidising agents in aqueous solution. Ti2+, V2+ and Cr2+ are strong reducing agents that can liberate hydrogen from dilute acids. The stability of half-filled d5 (Mn2+) and completely filled d10 (Zn2+) configurations is reflected in their Eo values.

Transition metals and their compounds are either diamagnetic (repelled by magnetic field, all electrons paired) or paramagnetic (attracted by magnetic field, unpaired electrons present). Ferromagnetism is an extreme form of paramagnetism. For first-row transition metal compounds, the orbital angular momentum contribution is quenched and the magnetic moment is determined by the spin-only formula: µ = √n(n+2) BM, where n is the number of unpaired electrons.

The magnetic moment increases with the number of unpaired electrons — a single unpaired electron gives 1.73 BM. Mn2+ (d5, 5 unpaired electrons) has the highest calculated magnetic moment of 5.92 BM. The measured magnetic moment thus gives a reliable indication of the number of unpaired electrons in an ion.

Transition metal ions are coloured because d-d electronic transitions absorb visible light; the complementary colour is observed. Ions with d0 (Sc3+, Ti4+) and d10 (Zn2+, Cu+) configurations are colourless. Complex compounds are formed because transition metal ions have small size, high ionic charge and available d orbitals for bonding. Examples include [Fe(CN)6]3–, [Cu(NH3)4]2+ and [PtCl4]2–.

Transition metals act as catalysts due to their ability to adopt multiple oxidation states and form complexes. Examples are V2O5 (Contact Process), Fe (Haber Process) and Ni (hydrogenation of fats). Interstitial compounds are formed by entrapment of small atoms (H, C, N) in the crystal lattice. They are non-stoichiometric, chemically inert, very hard, have high melting points, and retain metallic conductivity. Alloy formation is also common due to similar atomic radii.

Potassium dichromate (K2Cr2O7) is prepared by fusing chromite ore (FeCr2O4) with sodium carbonate in air to form sodium chromate, which is acidified to give sodium dichromate, then treated with KCl. Chromate (CrO42–, tetrahedral, yellow) and dichromate (Cr2O72–, two tetrahedra sharing a corner, orange) are interconvertible with change in pH. In acidic solution, Cr2O72– acts as a strong oxidising agent: Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O (Eo = 1.33 V), oxidising Fe2+ to Fe3+, I– to I2, Sn2+ to Sn4+, and H2S to S.

Potassium permanganate (KMnO4) is prepared commercially by alkaline oxidative fusion of MnO2 followed by electrolytic oxidation of manganate (MnO42–). It forms dark purple crystals isostructural with KClO4. In acidic solution, MnO4– + 8H+ + 5e– → Mn2+ + 4H2O (Eo = +1.52 V). It oxidises oxalates to CO2, Fe2+ to Fe3+, nitrites to nitrates, and iodides to iodine. In neutral/alkaline solution, MnO2 is the reduction product.

The lanthanoids (La to Lu, Z = 57–71) have the general electronic configuration [Xe] 4fn 5d0–1 6s2. All tripositive ions have the configuration 4fn (n = 0–14). The principal oxidation state is +3, though Ce4+ (stable due to noble gas configuration), Eu2+ and Yb2+ (stable due to f7 and f14 configurations) are also known. Lanthanoid contraction — the steady decrease in atomic and ionic radius from La to Lu due to poor shielding of 4f electrons — has far-reaching effects on the 5d transition metals.

Lanthanoids are silvery white soft metals that tarnish rapidly in air, are good conductors and react with water and dilute acids. Many Ln3+ ions are coloured and paramagnetic due to f electrons. The best-known application is in alloy steels; mischmetall (95% lanthanoid, 5% Fe) is used in making bullets, shells and lighter flints. Mixed Ln oxides serve as catalysts in petroleum cracking.

The actinoids (Ac to Lr, Z = 89–103) have the general electronic configuration [Rn] 5fn 6d0–1 7s2. Unlike lanthanoids, the 5f, 6d and 7s orbitals have comparable energies, leading to a wide range of oxidation states (+3 to +7 for early actinoids). The maximum oxidation state increases from +4 in Th to +7 in Np, then decreases. All actinoids are radioactive, making their study difficult. Actinoid contraction is greater per element than lanthanoid contraction due to poorer shielding by 5f electrons.

Early actinoids (Th, Pa, U, Np) show higher oxidation states and more complex chemistry than later members. The 5f electrons, unlike 4f electrons, are not deeply buried and can participate in bonding. Actinoids are highly reactive, attacked by HCl but passivated by HNO3 due to protective oxide layers. Their magnetic properties are more complex than those of lanthanoids.

Transition metals and their compounds have extensive industrial and technological applications. Iron and steel (with alloying metals Cr, Mn, Ni, V, W, Mo) form the most important structural materials. TiO2 is used as a white pigment, MnO2 in dry cells, and Zn and Ni/Cd in batteries. Coinage metals (Cu, Ag, Au) remain economically significant.

Catalytic applications include V2O5 in the Contact Process (SO2 to SO3), Fe in the Haber Process (N2 + H2 → NH3), Ni in catalytic hydrogenation of fats, PdCl2 in the Wacker Process (ethyne to ethanal), and TiCl4/Al(CH3)3 (Ziegler catalysts) in polyethylene manufacture. The photographic industry uses AgBr for its light-sensitive properties. Lanthanoid oxides are used as phosphors in TV screens.

Chapter 4 establishes the d- and f-block elements as a chemically rich and industrially vital group. The presence of partly filled d or f orbitals underpins the characteristic properties of these elements: variable oxidation states, paramagnetism, formation of coloured ions, catalytic behaviour, complex formation, and alloy and interstitial compound formation. The lanthanoid contraction is a particularly far-reaching consequence of f-orbital filling that explains the similarity between 4d and 5d transition metals.

From the board examination perspective, this chapter carries significant weightage. Students should be thorough with electronic configurations and their exceptions (Cr, Cu), the preparation and reactions of K2Cr2O7 and KMnO4, the spin-only magnetic moment formula, and the comparative study of lanthanoids vs actinoids. Understanding the relationship between electronic configuration, stability of oxidation states, and electrode potentials is essential for solving both short answer and application-based questions.