The d- and f-Block Elements

• Introduction & Position in Periodic Table

  • Transition elements (d-block): Groups 3-12
  • Inner transition elements (f-block): Lanthanoids and Actinoids at the bottom
  • General electronic configuration: $(n-1)d^{1-10}ns^{1-2}$

• The d-Block Elements (Transition Metals)

  • Series
    • 3d series: Scandium ($Sc$) to Zinc ($Zn$)
    • 4d series: Yttrium ($Y$) to Cadmium ($Cd$)
    • 5d series: Lanthanum ($La$), Hafnium ($Hf$) to Mercury ($Hg$)
    • 6d series: Actinium ($Ac$), Rutherfordium ($Rf$) to Copernicium ($Cn$)
  • General Properties
    • Atomic & Ionic Sizes
      • Gradual decrease across a period (shielding effect balances nuclear charge)
      • Atomic radii increase down the group (3d < 4d $\approx$ 5d due to Lanthanoid contraction)
    • Ionization Enthalpies
      • Intermediate between s- and p-block elements
      • Increases across a period with irregular trends
    • Oxidation States
      • Variable oxidation states due to small energy gap between $(n-1)d$ and $ns$ orbitals
      • Max oxidation state corresponds to total number of valence electrons (e.g., $Mn$ shows $+7$)
    • Magnetic Properties
      • Mostly paramagnetic due to unpaired d-electrons
      • Magnetic moment calculated using spin-only formula: $\mu = \sqrt{n(n+2)}$ BM
    • Color of Ions
      • Due to d-d electronic transitions absorbing visible light
      • Ions with $d^0$ or $d^{10}$ configurations are usually colorless
    • Formation of Colored & Complex Compounds
      • High nuclear charge, small size, and vacant d-orbitals enable complex formation (coordination compounds)
    • Catalytic Properties
      • Exhibit variable oxidation states and large surface area (e.g., $V_2O_5$, $Fe$, $Ni$)
    • Interstitial Compounds
      • Formed when small atoms ($H, C, N$) are trapped in the crystal lattice
    • Alloy Formation
      • Atoms with similar radii substitute each other (e.g., brass, bronze)

• Some Important Compounds of Transition Elements

  • Potassium Dichromate ($K_2Cr_2O_7$)
    • Preparation: From chromite ore ($FeCr_2O_4$)
    • Properties: Strong oxidizing agent in acidic medium
    • Structure: Dichromate ion consists of two tetrahedral $CrO_4$ sharing one corner
  • Potassium Permanganate ($KMnO_4$)
    • Preparation: Fusion of $MnO_2$ with $KOH$ and $O_2$, followed by electrolytic oxidation
    • Properties: Dark purple crystals, powerful oxidizing agent (acidic, neutral, alkaline media)

• The f-Block Elements (Inner Transition Elements)

  • Lanthanoids (4f-series)
    • Elements from Cerium ($Ce, 58$) to Lutetium ($Lu, 71$)
    • Common oxidation state: $+3$ (some show $+2$ and $+4$)
    • Lanthanoid Contraction: Steady decrease in atomic and ionic radii with increasing atomic number
      • Consequences: Similarity in properties of 4d and 5d series elements, difficulty in separation
  • Actinoids (5f-series)
    • Elements from Thorium ($Th, 90$) to Lawrencium ($Lr, 103$)
    • Radioactive nature
    • Greater range of oxidation states due to comparable energies of $5f, 6d,$ and $7s$ orbitals
    • Actinoid contraction is more pronounced than lanthanoid contraction

• Comparison: Lanthanoids vs Actinoids

  • Binding energy of 4f electrons is higher than 5f electrons
  • Actinoids show more variable oxidation states
  • Most actinoids are synthetic and radioactive, unlike lanthanoids (mostly natural except Promethium)