Comprehensive Notes on d- and f-Block Elements

Position and Classification

• d-Block (Groups 3–12)
  • Four horizontal series in which the final electron enters an (n-1)d orbital: 3d (Sc ➜ Zn), 4d (Y ➜ Cd), 5d (La,Hf ➜ Hg) and 6d (Ac, Rf ➜ Cn).
  • Elements often called transition metals when they possess at least one stable species whose $d$ subshell is incompletely filled.
  • Zn, Cd, Hg (ground-state configuration $d^{10}s^{2}$ and common oxidation state $d^{10}$) are studied with the block but are not transition elements by IUPAC definition.
• f-Block
  • 4f (Ce ➜ Lu) = lanthanoids; 5f (Th ➜ Lr) = actinoids.
  • Often called inner-transition metals; printed separately at table bottom.

Electronic Configurations

• General outer configuration for d-block: $(n-1)d^{1\text{–}10}ns^{1\text{–}2}$
  • Small $\Delta E$ between $ns$ and $(n-1)d$ ➜ many exceptions (e.g.
    $\text{Cr: }[Ar]3d^{5}4s^{1}$, $\text{Cu: }[Ar]3d^{10}4s^{1}$).
• For lanthanoids: common core $[Xe]$ with progressive filling of $4f$; stable aqueous ion $\text{Ln}^{3+}:4f^{n}$.
• For actinoids: common core $[Rn]$ with progressive filling of $5f$ (and some 6d participation); stable ion $\text{An}^{3+}$.

General Metallic Properties (d-Block)

• All (except Hg, Zn, Cd, Mn) show typical metallic lattice (bcc, hcp, ccp) giving:
  • high tensile strength, hardness, ductility
  • high electrical & thermal conductivities
  • metallic lustre & low volatility

Trends in Physical Properties

1. Melting Points & Enthalpy of Atomisation

• High $T<em>m$ and $\Delta H</em>a$ because many valence electrons (both $ns$ + $d$) participate in metallic bonding ➜ strong $M–M$ interactions.
• $T_m$ rises to maximum near $d^{5}$ (except anomalous Mn, Tc), then falls.
• $\Delta H_a$ peaks mid-series ➜ metals with very high atomisation enthalpy (e.g. W, Re, Os, Ir) are chemically noble.

2. Atomic / Ionic Radii

• Across a given series: gradual decrease (poor shielding by $d$ electrons).
• Between series: 4d radii > 3d; 5d ≈ 4d because of lanthanoid contraction (fill 4f before 5d ➜ extra nuclear charge not fully shielded).
• Consequence: Zr (160 pm) ≈ Hf (159 pm) ➜ occur together and hard to separate.

3. Density

• $\rho$ increases from Ti ➜ Cu due to combined rise in atomic mass and lanthanoid contraction.

Ionisation Enthalpies (IE)

• 1st IE increases only slightly across 3d row; small variation due to shielding of 4s by 3d.
• 2nd & 3rd IE show larger jumps; breaks at configurations yielding extra stability (e.g. $\text{Mn}^{2+}(d^{5})$, $\text{Zn}^{2+}(d^{10})$).

Oxidation States

• Variable; successive states often differ by +1 because electrons are removed first from $ns$ then $d$.
• Early elements (Sc, Ti, V, Cr, Mn) reach high states equal to group number (e.g. $\text{Cr}^{6+}$ in $\text{CrO}<em>4^{2-}$, $\text{Mn}^{7+}$ in $\text{MnO}</em>4^{-}$).
• Later elements favour low states: $\text{Fe}^{2+/3+}, \text{Co}^{2+/3+}, \text{Ni}^{2+}, \text{Cu}^{+ /2+}, \text{Zn}^{2+}$.
• Stability trend linked to $d^{0}, d^{5}, d^{10}$ configurations.

Standard Electrode Potentials & Redox Behaviour

• $E^{\circ}(\text{M}^{2+}/\text{M})$ generally becomes less negative left ➜ right (harder to oxidise), but Mn, Ni, Zn deviate owing to $d^{5}/d^{10}$ stability and hydration enthalpy.
  $\text{Cu}^{2+}/\text{Cu}: E^{\circ}=+0.34\,\text{V}$ ➜ cannot displace $\text{H}_2$ from acids.
• $\text{Cr}^{2+}, \text{V}^{2+}, \text{Ti}^{2+}$ are strong reductants; $\text{Mn}^{3+}, \text{Co}^{3+}$ strong oxidants.

Magnetic Properties

• Origin: unpaired $d$ (or $f$) electrons; spin-only formula $\mu = \sqrt{n(n+2)}\;\text{BM}$.
• $d^{0}$/$d^{10}$ ions (Sc$^{3+}$, Zn$^{2+}$) are diamagnetic; others paramagnetic.
• 1 unpaired e⁻ ➜ $1.73\,\text{BM}$; 5 unpaired e⁻ ➜ $5.92\,\text{BM}$.

Coloured Ions

• Colour arises from $d\,\rightarrow\,d$ transitions between split $d$-orbitals; energy equals visible-light frequency.
• Observed colour depends on ligand field (e.g. \text{[Ti(H2O)6]^{3+}} purple, $\text{Cu}^{2+}$ blue).

Complex Formation

• High charge-to-radius ratio + vacant $d$ orbitals ➜ strong tendency to form complexes: $[Fe(CN)<em>6]^{3-/4-}, [Cu(NH</em>3)<em>4]^{2+}, [PtCl</em>4]^{2-}$.

Catalytic Properties

• Multiple oxidation states + ability to adsorb reactants on surface.
• Examples:
  • $\text{V}<em>2\text{O}</em>5$ in Contact process (SO$<em>2$ ➜ SO$</em>3$)
  • Fe in Haber synthesis (N$<em>2$+3H$</em>2$ ➜ 2NH$<em>3$) • Ni in catalytic hydrogenation of alkenes. • $\text{Fe}^{3+}$ catalyses $\text{S}</em>2\text{O}_8^{2-}+I^{-}$ reaction.

Interstitial Compounds

• Small non-metal atoms occupy holes in metal lattice: $\text{TiC}, \text{Mn}<em>4\text{N}, \text{Fe}</em>3\text{H}, \text{VH}_{0.56}$.
• Features: very hard, high $T_m$, retain metallic conductivity, chemically inert.

Alloy Formation

• Similar atomic radii (±15 %) facilitate solid solutions: steel alloys (Fe–Cr, Fe–V, Fe–Mn, Fe–Ni), brass (Cu–Zn), bronze (Cu–Sn), stainless steel (Fe–Cr–Ni).

Important Compounds of 3d Series

1. Potassium Dichromate, $\text{K}<em>2\text{Cr}</em>2\text{O}_7$

• Prepared via fusion of chromite $\text{FeCr}<em>2\text{O}</em>4$ with $\text{Na}<em>2\text{CO}</em>3/O<em>2$ ➜ $\text{Na}</em>2\text{CrO}<em>4$, acidified to $\text{Na}</em>2\text{Cr}<em>2\text{O}</em>7$ then precipitated with $\text{KCl}$.
• $\text{CrO}<em>4^{2-}+2H^{+} \rightleftharpoons \text{Cr}</em>2\text{O}<em>7^{2-}+H</em>2O$ (pH-dependent interconversion).
• Strong oxidant in acid: $\text{Cr}<em>2\text{O}</em>7^{2-}+14H^{+}+6e^{-}\;\rightarrow\;2Cr^{3+}+7H<em>2O$ ( $E^{\circ}=1.33\,V$ ). • Oxidises $I^{-}\rightarrow I</em>2, Fe^{2+}\rightarrow Fe^{3+}, Sn^{2+}\rightarrow Sn^{4+}, H_2S\rightarrow S$.

2. Potassium Permanganate, $\text{KMnO}_4$

• Producing steps: $\text{MnO}<em>2$ + $KOH$/$O</em>2$ ➜ $K<em>2\text{MnO}</em>4$ (green), electrolytic or acid disproportionation ➜ $\text{KMnO}_4$ (purple).
• Redox potentials (acidic):
  $\text{MnO}<em>4^{-}+8H^{+}+5e^{-}\;\rightarrow\; Mn^{2+}+4H</em>2O \; (E^{\circ}=1.52\,V)$.
• Oxidises $Fe^{2+}, I^{-}, NO<em>2^{-}, C</em>2O<em>4^{2-}, SO</em>3^{2-}, H<em>2S$; medium controls products (MnO$</em>2$ in neutral, MnO$_4^{2-}$ in strong base).

Lanthanoids (4f Series)

Electronic Configuration

• $[Xe]6s^{2}4f^{1\text{–}14}5d^{0\text{–}1}$; tripositive ion $\text{Ln}^{3+}:4f^{n}$ ((n=1–14)).

Lanthanoid Contraction

• Poor shielding by 4f ➜ steady decrease in $r<em>{\text{atom}}$ and $r</em>{\text{ion}}$ La ➜ Lu.
• Consequences:
  • 5d radii ≈ 4d ➜ Zr/Hf chemical similarity.
  • Rising density, hardness.

Oxidation States

• Predominantly $+3$.
• $+4$: Ce, Pr, Tb; strong oxidants (e.g. $\text{Ce}^{4+}$ in $\text{Ce(SO}<em>4)</em>2$).
• $+2$: Eu (stable, reductant, $f^{7}$), Yb ($f^{14}$), Sm.

General Chemistry

• Soft, silvery; tarnish quickly; form $\text{Ln}<em>2\text{O}</em>3, Ln(OH)<em>3, LnX</em>3$.
• React with H$<em>2$, C, N, S on heating ➜ $LnH</em>2\text{–}LnH<em>3, LnC</em>2, LnN, Ln<em>2S</em>3$.

Uses

• Misch-metal (~95 % lanthanoid + 5 % Fe) in Mg alloys, lighter flints.
• Ln oxides as petroleum-cracking catalysts; Eu$^{3+}$, Tb$^{3+}$ phosphors in TV screens.

Actinoids (5f Series)

Electronic Configuration

• $[Rn]7s^{2}5f^{1\text{–}14}6d^{0\text{–}1}$; irregular due to near-degenerate 5f, 6d, 7s.

Actinoid Contraction

• Poor shielding by 5f ➜ sharper radius decline than lanthanoids.

Oxidation States

• Wider range: $+3$ predominant, but $+4$–$+7$ common in early members.
  • Th $+4$; Pa $+5$; U $+6$ (e.g. $\text{UO}_2^{2+}$); Np $+7$.
• Later actinoids revert to $+3$ (Am, Cm…).

General Properties

• All radioactive; early members have long half-lives, later (Bk–Lr) short (minutes–days).
• More chemically reactive than lanthanoids; form oxides $\text{AnO}_2$, hydrides, halides.
• 5f orbitals less buried ➜ greater participation in bonding ➜ complex redox chemistry, organometallics.

Comparative Highlights

• Electronic configuration: lanthanoids = tidy 4f filling; actinoids have 5f/6d intermixing.
• Ionic radii: both show contraction, but actinoid contraction larger per step.
• Oxidation states: lanthanoids mostly $+3$; actinoids multiple high states.
• Chemical reactivity: lanthanoids resemble alkaline-earth metals; actinoids more electropositive and radioactive.

Applications of d- & f-Block Elements

• Structural: steels (Fe-Cr, Fe-V, Fe-Mn, Fe-Ni), stainless steel, super-alloys (Ni-Cr).
• Catalysts: $V<em>2O</em>5$ (SO$<em>2$ ➜ SO$</em>3$), Fe (NH$<em>3$), Ni (hydrogenation), PdCl$</em>2$ (Wacker), Ziegler–Natta (TiCl$<em>4$/AlR$</em>3$).
• Pigments & Materials: $TiO<em>2$ (white), $MnO</em>2$ in batteries, AgBr in photography.
• Energy: Th, Pa, U supply nuclear fuel.

Selected Equations & Formulae

• Spin-only magnetic moment: $\mu(\text{BM})=\sqrt{n(n+2)}$.
• Example reduction potentials: $\text{Cr}<em>2\text{O}</em>7^{2-}+14H^{+}+6e^{-}\rightarrow 2Cr^{3+}+7H_2O$ $\,(E^{\circ}=1.33\,V)$.
• Disproportionation: $2Cu^{+}\;(aq)\;\longrightarrow\;Cu^{2+}\;(aq)+Cu\;(s)$.

Ethical, Environmental & Practical Notes

• Transition-metal catalysts lower energy consumption in industry, aiding sustainability.
• Heavy-metal toxicity (Cr(VI), Cd, Hg) necessitates strict environmental controls.
• Radioactive actinoids (U, Pu) provide vast nuclear energy but pose waste-management and proliferation challenges.