Transition and Inner-Transition Elements

Overview

• Covers d-block (transition) and f-block (inner transition) elements, their positions, electronic configurations, general properties and important compounds.
• Emphasises trends in the first transition series (3d), lanthanoid and actinoid series, and significant applications (alloys, catalysts, pigments, batteries).

Position And Electronic Configurations

• d-block: Groups 3–12; (n-1)d progressively filled in 3d, 4d, 5d, 6d series.
• f-block: Lanthanoids (4f) and actinoids (5f) placed separately.
• IUPAC transition-metal definition: incomplete d subshell in neutral atom or ions; Zn, Cd, Hg (d10 ns2) usually not classed as transition metals.
• General outer-electron configuration: (n-1)d1–10 ns1–2 (many exceptions due to small energy differences and stability of half/full shells).
• Examples of exceptions: Cr: 3d5 4s1; Cu: 3d10 4s1; Pd: 4d10 5s0.

Key Concepts And Definitions

• Lanthanoid Contraction: steady decrease in atomic/ionic radii across 4f series; causes similar radii for 4d and 5d congeners (e.g., Zr and Hf).
• Actinoid Contraction: analogous decrease across 5f series; generally larger element-to-element contraction than lanthanoids.
• Interstitial Compounds: non-stoichiometric compounds formed when small atoms (H, C, N) occupy metal lattice interstices (e.g., TiC, VH0.56).
• Disproportionation: a species in intermediate oxidation state converts into both higher and lower oxidation states (example: 2MnO4^2- + 4H+ → 2MnO4^- + MnO2 + 2H2O).

General Properties Of Transition Metals (d-Block)

• Typical metallic properties: high tensile strength, ductility, malleability, electrical/thermal conductivity, metallic lustre.
• High melting/boiling points (maxima near d5); high enthalpies of atomisation due to involvement of (n-1)d electrons in metallic bonding.
• Crystal structures vary (hcp, bcc, ccp); structure trends across series listed for many 3d, 4d, 5d elements.
• Atomic/ionic sizes: gradual decrease across a series; 4f filling leads to lanthanoid contraction so 4d and 5d radii are similar.
• Densities generally increase across a series due to decreasing radius with increasing mass.

Ionisation, Oxidation States And Electrode Potentials

• Successive ionisation enthalpies do not increase as steeply as main-group elements; ns electrons lost before (n-1)d on ion formation.
• Variety of oxidation states arises from partially filled d orbitals; oxidation states commonly change by one.
• Typical first-row common oxidation states listed (common ones include +2, +3; Mn shows +2 to +7).
• Standard electrode potentials E°(M2+/M) vary irregularly (Cu is +0.34 V due to high atomisation enthalpy and hydration balance).
• E° values influenced by sublimation enthalpy, ionisation enthalpies and hydration enthalpy.

Magnetic Properties And Colours

• Paramagnetism arises from unpaired electrons; diamagnetism if all electrons paired.
• For most first-row hydrated ions, orbital angular momentum is quenched; spin-only formula used: μ = √(n(n+2)) BM where n = number of unpaired electrons.
• Experimental magnetic moments generally align with spin-only values; deviations occur due to orbital contributions.
• Coloured ions: d–d transitions absorb visible light; colour depends on d-electron count and ligand field (example: Cu2+ is blue, Mn2+ is pink, Zn2+ is colourless).

Complex Formation, Catalysis, Interstitial Compounds And Alloys

• Transition metals readily form complexes due to small ionic sizes, high charges and available d orbitals.
• Catalytic activity linked to multiple accessible oxidation states and complex formation (examples: V2O5 for SO2 oxidation, Fe for Haber process, Ni for hydrogenation).
• Interstitial compounds: small atoms trapped in metal lattices; high melting points, hardness, metallic conductivity.
• Alloys: readily formed among transition metals due to similar radii; important steels and special alloys (mischmetal contains lanthanoids).

Important Compounds: Preparation, Properties, Uses

• Potassium Dichromate (K2Cr2O7)

  • Prepared from chromite (FeCr2O4) fused with carbonate and oxidised; chromate → dichromate upon acidification.
  • Chromate/dichromate equilibrium depends on pH; dichromate in acidic medium: Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O (E° = 1.33 V).
  • Uses: oxidising agent in organic chemistry, leather industry, volumetric primary standard.

• Potassium Permanganate (KMnO4)

  • Prepared by alkaline fusion of MnO2 → manganate (green) then electrolytic oxidation to permanganate (purple).
  • Permanganate redox depends on pH:
    • In acid: MnO4^- + 8H+ + 5e- → Mn2+ + 4H2O (E° = +1.52 V).
    • In neutral/alkaline: other reduction products (MnO2 or MnO4^2-).
  • Uses: strong oxidant in analytical and preparative chemistry, bleaching textiles, decolourising oils.
  • Important reactions: oxidation of I-, Fe2+, oxalate, H2S, SO2, NO2- (equations provided in lecture).

Lanthanoids (4f Series)

• General features:
  • Electronic: common outer configuration 6s2; varying 4f occupancy (4f1–4f14).
  • Most stable oxidation state: +3; occasional +2 or +4 for some elements (Eu2+, Yb2+, Ce4+, Tb4+).
  • Lanthanoid contraction causes steady decrease in atomic/ionic radii across series.
• Properties:
  • Silvery-white soft metals; progressively harder with increasing atomic number.
  • Ionic radii and magnetic properties follow 4f electron counts; many trivalent ions are coloured.
  • Reactivity: react with water, acids; form oxides (M2O3), hydroxides M(OH)3, carbides, halides.
• Applications: alloy steels (mischmetal), catalysts for petroleum cracking, phosphors for displays.

Actinoids (5f Series)

• General features:
  • Radioactive series from Th to Lr; electronic configurations involve 5f, 6d, 7s orbitals.
  • Exhibit wider range of oxidation states than lanthanoids; early actinoids show higher oxidation states (e.g., Pa +5, U +6, Np +7).
  • Actinoid contraction: decrease in size across series, often larger per-element change than lanthanoids.
• Properties:
  • Highly reactive metals; combine with nonmetals, react with water, form oxides and hydrides.
  • Magnetic and chemical behaviour more complex due to 5f orbital involvement in bonding.
• Challenges: radioactivity and short half-lives limit study and applications.

Applications Of d- And f-Block Elements

• Structural materials: iron and steels (alloying with Cr, Mn, Ni, Mo, W).
• Catalysts: V2O5 (SO2 → SO3), Fe (Haber), Ni (hydrogenation), PdCl2 (Wacker process), Ziegler catalysts (polyethylene).
• Pigments and ceramics: TiO2 (white pigment), MnO2 (dry cells).
• Batteries and electrochemical uses: Zn, Ni-Cd cells.
• Photographic industry: AgBr.

Summary (Key Points)

• d-block elements: partially filled d orbitals => variable oxidation states, coloured ions, complexes, catalytic activity, high melting/boiling points.
• Lanthanoids: mainly +3 oxidation state; lanthanoid contraction affects properties of subsequent elements.
• Actinoids: radioactive, complex oxidation-state behaviour; 5f electrons more available for bonding than 4f.
• Important oxidants and reagents: K2Cr2O7 and KMnO4 — preparation routes and redox behaviour depend strongly on pH and conditions.

Key Terms And Definitions

• Transition Metal: metal with incomplete d subshell in atom or its ions.
• Lanthanoid Contraction: gradual decrease in ionic/atomic radii across 4f series.
• Interstitial Compound: metal lattice containing small atom in interstitial sites.
• Spin-Only Magnetic Moment: μ = √(n(n+2)) BM (n = unpaired electrons).

Action Items / Exercises (Assigned Problems From Lecture)

• Practice electronic configurations and oxidation states for specified ions (e.g., Cr3+, Cu+, Pm3+, Ce4+).
• Explain stability trends (e.g., Mn2+ vs Fe2+, highest oxidation states in oxides/fluorides).
• Prepare stepwise accounts for K2Cr2O7 from chromite and KMnO4 from MnO2.
• Calculate magnetic moments for given ions (use spin-only formula).
• Answer conceptual questions on ionisation enthalpy irregularities, electrode potentials and catalytic mechanisms.