d-Block & f-Block Elements

The d-block elements (transition metals) occupy groups 3-12 in periods 4-7 of the periodic table. They are characterised by partially filled d-orbitals in their ground state or common oxidation states. The general electronic configuration is (n-1)d^(1-10) ns^(0-2).

First Row Transition Metals (3d series: Sc to Zn)

Electronic Configurations: Most follow the expected filling order, with two key exceptions:

• Cr: [Ar] $3d^{5} \; 4s^{1}$ (not $3d^{4} \; 4s^{2}$) — extra stability of half-filled d-subshell
• Cu: [Ar] $3d^{10} \; 4s^{1}$ (not $3d^{9} \; 4s^{2}$) — extra stability of completely filled d-subshell

Anomalous configurations also occur in heavier elements: Mo ($4d^{5} \; 5s^{1}$), Ag ($4d^{10} \; 5s^{1}$), Pd ($4d^{10} \; 5s^{0}$), Pt ($5d^{9} \; 6s^{1}$), Au ($5d^{10} \; 6s^{1}$).

General Properties of d-Block Elements

Variable oxidation states: Due to small energy difference between (n-1)d and ns electrons, multiple oxidation states are possible. The maximum oxidation state increases from Sc (+3) to Mn (+7), then decreases. Common trends:

• +2 state is most common (loss of $ns^{2}$ electrons)
• Higher oxidation states are stabilised by fluoride and oxide ligands
• Mn shows maximum oxidation state (+7) in the 3d series
• Stability of +2 state increases across the series (increasing nuclear charge holds d-electrons tighter)

Metallic character: All are metals with high melting points, boiling points, and densities. Metallic bonding involves both ns and (n-1)d electrons. Trends: melting point increases to middle of series (maximum at Cr or Mo) then decreases. Exception: Mn has anomalously low melting point (weak metallic bonding due to complex crystal structure).

Atomic and ionic radii: Gradual decrease across the series (poor shielding by d-electrons), then slight increase at the end. The contraction is much less than across s- or p-blocks. First three elements (Sc, Ti, V) show significant decrease; middle elements are nearly constant; Cu and Zn show slight increase.

Ionisation enthalpy: Generally increases across the series but not regularly. Irregularities due to extra stability of $d^{0}$, $d^{5}$, and $d^{10}$ configurations. IE values are intermediate between s-block (low) and p-block (high).

Colour: Most transition metal ions are coloured due to d-d transitions. $d^{0}$ ($Sc^{3}$+, $Ti^{4}$+) and $d^{10}$ (Cu⁺, $Zn^{2}$+) are colourless. $Cu^{2}$+ ($d^{9}$) is blue, $Fe^{3}$+ ($d^{5}$) is yellow, $Cr^{3}$+ ($d^{3}$) is green, $Mn^{2}$+ ($d^{5}$) is very pale pink.

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Magnetic properties: Paramagnetic when unpaired d-electrons present. Magnetic moment related to number of unpaired electrons: $\mu$ = $\sqrt{n(n+2}$) BM.

Catalytic activity: Transition metals are excellent catalysts due to: (1) variable oxidation states allowing electron transfer, (2) ability to form intermediates with reactants, (3) large surface area in finely divided state. Examples: Fe in Haber process, V₂O₅ in Contact process, Ni in hydrogenation, MnO₂ in decomposition of KClO₃.

Interstitial compounds: Small atoms (H, C, N, B) occupy voids in metal lattice, forming hard, high-melting, chemically inert compounds. TiC, steel (Fe-C), TiN are examples.

Alloy formation: Similar atomic sizes allow substitution in crystal lattices. Brass (Cu-Zn), bronze (Cu-Sn), stainless steel (Fe-Cr-Ni).

Important Compounds

Potassium dichromate (K₂Cr₂O₇): Orange crystals, strong oxidising agent in acidic medium.
$Cr2O7^{2}$- + 14H⁺ + 6e^- → $2Cr^{3}$+ + 7H₂O (E° = +1.33 V). Used in volumetric analysis, leather tanning. In acidic medium: orange ($Cr2O7^{2}$-); in basic medium: yellow ($CrO4^{2}$-). Equilibrium: $2CrO4^{2}$- + 2H⁺ ⇌ $Cr2O7^{2}$- + H₂O.

Potassium permanganate (KMnO₄): Purple crystals, powerful oxidising agent. In acidic medium: MnO₄^- + 8H⁺ + 5e^- → $Mn^{2}$+ + 4H₂O. In neutral/weakly alkaline: MnO₄^- + 2H₂O + 3e^- → MnO₂ + 4OH^-. Self-indicator in titrations (pink to colourless). Prepared from pyrolusite (MnO₂) by fusion with KOH and oxidation.

f-Block Elements

Lanthanoids (La to Lu, $4f^{0}$-14 $5d^{0}$-1 $6s^{2}$): Silvery-white metals with +3 as dominant oxidation state. Lanthanoid contraction: Steady decrease in atomic/ionic radii across the series due to poor shielding by 4f electrons. Consequences: (1) similar radii for 4d and 5d elements in same group, (2) difficulty in separation of lanthanoids, (3) basicity decreases La(OH)₃ to Lu(OH)₃.

Anomalous oxidation states: $Ce^{4}$+ ($4f^{0}$, noble gas-like), $Eu^{2}$+ ($4f^{7}$, half-filled), $Yb^{2}$+ ($4f^{14}$, completely filled).

Actinoids (Ac to Lr, $5f^{0}$-14 $6d^{0}$-1 $7s^{2}$): Radioactive, many are synthetic. Show greater range of oxidation states (+3 to +7) than lanthanoids because 5f, 6d, and 7s have comparable energies. U, Np, Pu are most important. Actinoid contraction is greater than lanthanoid contraction due to poorer shielding by 5f electrons.

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