d - and f- BLOCK ELEMENTS

Transition metals and inner transition metals occupy the "bridge" region of the periodic table, and their chemistry — variable oxidation states, coloured ions, magnetism, catalysis — is a reliable source of 1–2 NEET questions every year. The topic rewards students who understand why these elements behave differently from s- and p-block metals rather than those who only memorise lists.

Position, Classification and Electronic Configurations

The d-block comprises the elements in Groups 3–12, filling their (n−1)d subshell while the outermost ns subshell already contains electrons. There are four such series:

1. 3d series (first transition series): Sc → Zn
2. 4d series (second): Y → Cd
3. 5d series (third): La, Hf → Hg
4. 6d series (fourth, incomplete): Ac, Rf onwards

The general valence-shell notation is (n−1)d¹⁻¹⁰ ns¹⁻². A strict definition of a transition element requires a partially filled d subshell either in the free atom or in a common oxidation state. By this criterion Zn, Cd, Hg (d¹⁰s²) are not true transition metals — their d shell is complete in both the atom and the +2 ion — yet they are studied along with the block. Sc and Y, with d¹, are included even though Sc³⁺ is d⁰.

Two configurations in the 3d series break the expected pattern because a half-filled or fully-filled d subshell gains extra exchange energy and symmetry:

• Cr: 3d⁵ 4s¹ (not 3d⁴ 4s²)
• Cu: 3d¹⁰ 4s¹ (not 3d⁹ 4s²)

The f-block (inner transition elements) consists of two rows of 14 elements each, in which the antepenultimate (n−2)f subshell is progressively filled:

• Lanthanoids: Ce (Z = 58) to Lu (Z = 71), following La; general configuration 4f¹⁻¹⁴ 5d⁰⁻¹ 6s².
• Actinoids: Th (Z = 90) to Lr (Z = 103), following Ac; 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². All are radioactive; elements beyond uranium are man-made transuranium elements.

Physical Properties and Trends Across a Series

Transition metals are typically hard, high-melting, high-boiling, lustrous, malleable metals with good electrical and thermal conductivity. The strength of metallic bonding here comes from both ns and (n−1)d electrons participating in the bonding, so metals with the maximum number of unpaired d electrons available for bonding (e.g. Cr, Mo, W) have the highest melting points. Zn, Cd and Hg, with no unpaired d electrons, are soft and low-melting — mercury is a liquid at room temperature.

Atomic and ionic radii: Radii decrease from Sc to about Mn/Fe, then remain almost constant through Fe–Ni, and rise slightly at Cu and Zn. The near-constancy arises because the added d electron screens the nucleus fairly effectively, roughly balancing the increased nuclear charge; the final rise reflects strong electron–electron repulsion in the nearly complete d shell. Radii increase from 3d to 4d, but 5d radii are almost equal to the corresponding 4d values — a consequence of the lanthanoid contraction, the cumulative size shrinkage across the 4f series due to poor shielding by f electrons. This is why pairs like Zr–Hf and Nb–Ta are chemically almost twins and hard to separate.

Ionisation enthalpies increase irregularly along a series as nuclear charge grows, but the increases are gradual, not sharp, because the incoming electrons enter an inner shell. Zn has an anomalously high first ionisation enthalpy owing to its stable d¹⁰s² configuration.

Density increases across a series (mass rises faster than volume falls), making the middle and late transition metals among the densest known (Os, Ir).

Chemical Behaviour: Oxidation States, Colour, Magnetism, Catalysis

Variable oxidation states. Because (n−1)d and ns electrons have comparable energies, both are available for bonding, and successive oxidation states differ by one unit (unlike the p-block, where jumps of two are common). In the 3d series the maximum oxidation state rises to +7 at Mn (Mn: 3d⁵4s², so 2 + 5 = 7) and then falls, since after d⁵ the electrons become increasingly hard to remove. Sc shows only +3; Zn only +2.

• Low oxidation states form ionic, basic oxides (MnO, FeO); high oxidation states are covalent and acidic (Mn₂O₇, CrO₃).
• Oxo-anions such as MnO₄⁻, CrO₄²⁻, VO₄³⁻ display the group's highest oxidation state, stabilised by oxygen or fluorine.
• Among the heavier series, higher oxidation states are relatively more stable (e.g. Mo(VI) and W(VI) are far less oxidising than Cr(VI)).

Standard electrode potentials. Most 3d metals have negative E°(M²⁺/M) values and liberate H₂ from dilute acids. Copper is the exception (E° = +0.34 V) because its high sublimation and ionisation enthalpies are not compensated by hydration enthalpy. Mn²⁺ (d⁵), Fe³⁺ (d⁵) and Zn²⁺ (d¹⁰) show unusual stability; hence E° for Mn³⁺/Mn²⁺ and Co³⁺/Co²⁺ are strongly positive (these ions are powerful oxidants).

Colour. Ions with partly filled d orbitals absorb visible light through d–d electronic transitions — the d orbitals are split in energy by ligands, and the ion transmits the complementary colour of the light absorbed. Thus Cu²⁺ (d⁹) is blue, Ni²⁺ (d⁸) green, Mn²⁺ (d⁵) pale pink, while Sc³⁺ (d⁰), Ti⁴⁺ (d⁰), Cu⁺ (d¹⁰) and Zn²⁺ (d¹⁰) are colourless. (The intense colours of MnO₄⁻ and Cr₂O₇²⁻, where the metal is d⁰, arise from charge-transfer transitions, not d–d.)

Magnetic properties. Species with unpaired electrons are paramagnetic; those with all electrons paired are diamagnetic. For 3d ions the "spin-only" magnetic moment is μ = √[n(n+2)] BM, where n = number of unpaired electrons. So Ti³⁺ (d¹) gives 1.73 BM, V³⁺ (d²) 2.83 BM, Mn²⁺ (d⁵) 5.92 BM — the maximum in the series. Fe, Co, Ni are ferromagnetic in the metallic state.

Catalysis, alloys and interstitial compounds.

• Catalytic action stems from variable oxidation states (allowing electron transfer) and surface adsorption: Fe in Haber's process, V₂O₅ in the Contact process, TiCl₄ in Ziegler–Natta polymerisation, Ni in hydrogenation, MnO₂ in KClO₃ decomposition.
• Interstitial compounds form when small atoms (H, C, N, B) occupy voids in the metal lattice. They are hard, chemically inert, retain metallic conductivity, and are usually non-stoichiometric (e.g. TiC, Fe₃H, VH₀.₅₆, TiH₁.₇).
• Alloys form readily since the metals have similar radii and one can replace another in the lattice — brass (Cu–Zn), bronze (Cu–Sn), stainless steel (Fe–Cr–Ni), and mischmetal (~95% lanthanoid metal + Fe), used in Mg-based alloys and lighter flints.

Transition metals also form abundant complexes, favoured by small highly charged ions with vacant d orbitals of suitable energy.

Two Important Compounds: K₂Cr₂O₇ and KMnO₄

Potassium dichromate is prepared from chromite ore, FeCr₂O₄, in three stages:

1. Fusion of the ore with molten alkali/Na₂CO₃ in air gives sodium chromate.
2. Acidification of chromate gives sodium dichromate.
3. Treating sodium dichromate with KCl yields the less soluble orange K₂Cr₂O₇, which crystallises out.

Chromate (yellow, tetrahedral) and dichromate (orange) are interconverted by pH: alkali converts Cr₂O₇²⁻ → 2CrO₄²⁻, acid reverses it. Both contain Cr in +6, and the dichromate ion has two tetrahedra sharing one oxygen corner. In acidic medium dichromate is a strong oxidising agent:

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V)

It oxidises I⁻ → I₂, Fe²⁺ → Fe³⁺, H₂S → S, and Sn²⁺ → Sn⁴⁺.

Potassium permanganate is made from pyrolusite, MnO₂: fusion with KOH in the presence of air or KNO₃ gives green K₂MnO₄, which is then oxidised (by disproportionation in acid/neutral medium, or electrolytically) to purple KMnO₄. It is a dark purple, diamagnetic solid; MnO₄⁻ is tetrahedral, with the colour due to charge transfer.

Its oxidising action depends sharply on medium:

• Acidic: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.52 V)
• Neutral/faintly alkaline: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻
• Strongly alkaline: MnO₄⁻ + e⁻ → MnO₄²⁻

In acid solution it oxidises oxalic acid to CO₂, Fe²⁺ to Fe³⁺, I⁻ to I₂, and H₂S to sulphur; it is widely used in volumetric analysis as a self-indicator.

Lanthanoids and Actinoids

Lanthanoids. All are silvery-white, reactive metals. The dominant oxidation state is +3; +2 and +4 appear where they lead to f⁰, f⁷ or f¹⁴ configurations or nearly so — hence Ce⁴⁺ (f⁰, a useful oxidising agent in solution), Eu²⁺ (f⁷) and Yb²⁺ (f¹⁴). Ionic radii shrink steadily from La³⁺ to Lu³⁺ (lanthanoid contraction), making chemical separation of the individual elements notoriously difficult since their properties are so similar. Most Ln³⁺ ions are coloured and paramagnetic (La³⁺ and Lu³⁺, being f⁰ and f¹⁴, are colourless and diamagnetic). Because 4f electrons are buried deep beneath the filled 5s and 5p subshells, they barely participate in bonding and are hardly perturbed by neighbouring atoms or ligands; lanthanoid absorption bands are sharp and their magnetic moments match the free-ion spin-only-plus-orbital value unusually well — a sharp contrast with d-block ions, whose outermost d orbitals are strongly influenced by the surrounding ligand field. Lanthanoids are soft, electropositive, silvery metals that tarnish in moist air, burn in oxygen to the oxide Ln₂O₃, and liberate hydrogen from dilute acids and even from hot water; reactivity falls slightly as atomic number rises because growing effective nuclear charge binds the valence electrons a little more tightly.

Actinoids. All fifteen members, Ac through Lr, are radioactive, and only the earliest few (Th, Pa, U) occur in nature in workable amounts — the rest are made artificially in reactors or accelerators. As in the lanthanoids, +3 becomes the dominant oxidation state late in the series (from about Am onward), but the early actinoids show a much wider spread of states — Th(+4), Pa(+5), U(+6), Np and Pu reaching +7 in some compounds — because the 5f, 6d and 7s orbitals lie closer together in energy than the corresponding 4f, 5d, 6s orbitals of the lanthanoids, leaving more electrons available for bonding. This makes early actinoid chemistry noticeably more like d-block chemistry (variable oxidation states, complex formation, appreciable covalent character) than lanthanoid chemistry ever is. An actinoid contraction parallels the lanthanoid contraction and is in fact sharper, since 5f orbitals shield nuclear charge even less effectively than 4f orbitals do. Because most actinoids are scarce, intensely radioactive and studied only in milligram quantities, their chemistry is explored far less thoroughly — and tested far less often on NEET — than that of the lanthanoids.

Common Mistakes and Exam Traps

• Confusing "d-block" with "transition element." Zn, Cd and Hg belong to the d-block (they fill a d subshell while being built up) but fail the strict definition of a transition element, since their d subshell is completely filled in both the free atom and their common +2 ion. NEET often tests exactly this one-line distinction.
• Assuming every coloured species owes its colour to a d–d transition. The intense purple of MnO₄⁻ and orange of Cr₂O₇²⁻ arise from charge-transfer transitions even though the metal centre is d⁰ (which the usual rule would call colourless) — a classic "spot the exception" question.
• Reciting Cr and Cu's anomalous configurations without the reasoning. Many students know 3d⁵4s¹ and 3d¹⁰4s¹ by rote but cannot explain that the extra stability comes from symmetric charge distribution and exchange energy in a half-filled or fully-filled subshell — a reasoning-based question can expose this gap even when the answer is memorised correctly.
• Treating lanthanoid contraction and the near-identical radii of 4d/5d pairs (Zr–Hf, Nb–Ta) as two separate facts. They are the same phenomenon seen from two angles, and assertion–reason questions frequently link them.