d- and f-Block Elements

Transition Elements (d-block): Elements with partially filled $d$-orbitals in atom or in common oxidation states. Located in groups 3-12 of periodic table.

Four Series:

• 3d series (1st transition): Sc(21) to Zn(30)
• 4d series (2nd): Y(39) to Cd(48)
• 5d series (3rd): La(57), Hf(72) to Hg(80)
• 6d series (4th): Ac(89), Rf(104) onwards (mostly synthetic)

General Electronic Configuration: $(n-1)d^{1-10}\,ns^{1-2}$

3d Series Configurations:

Cr and Cu anomaly: Extra stability of half-filled ($d^5$) and fully-filled ($d^{10}$) configurations due to symmetric distribution and exchange energy.

Note: Zn ($d^{10}\,s^2$) and Hg are sometimes considered "non-transition" because their d-orbitals are fully filled and don't show typical transition properties. But traditionally still grouped under d-block.

Trends in Properties:

Reasons for Typical Transition Element Behavior:

• Variable oxidation states (multiple available d-electrons)
• Forms complex ions (vacant d-orbitals accept lone pairs)
• Colored compounds (d-d transitions)
• Magnetic properties (unpaired d-electrons)
• Catalytic activity (variable oxidation states)
• Alloy formation (similar atomic sizes)

Variable Oxidation States: Most transition metals show multiple ONs.

Maximum ON = total of $4s + 3d$ electrons (for early elements). E.g., Mn: $+7$ (in $MnO_4^-$); Cr: $+6$ (in $Cr_2O_7^{2-}$).

Stability trends:

• $+2$ stable in middle of series (V to Cu)
• $+3$ stable for early elements (Sc, Ti, V) and Fe, Co
• Higher ONs less stable down the group (for early transition metals); more stable for heavier (Pt, Ru, etc.)

Color in Transition Metal Compounds:

Color due to d-d transitions: electron moves from one $d$-orbital to another absorbing visible light; observed color is complementary.

Magnetic Properties:

• Diamagnetic: No unpaired electrons; weakly repelled (e.g., $Zn^{2+}$, $Cu^+$, $Cd^{2+}$)
• Paramagnetic: Unpaired electrons; attracted

Magnetic Moment ($\mu$): "Spin-only" formula (ignoring orbital contribution): $$\mu = \sqrt{n(n+2)} \text{ B.M.}$$ where $n$ = number of unpaired electrons, B.M. = Bohr Magneton ($9.27 \times 10^{-24}$ J/T).

Examples:

• $Fe^{2+}$ ($d^6$): 4 unpaired → $\mu = \sqrt{4 \times 6} = 4.9$ B.M.
• $Cu^{2+}$ ($d^9$): 1 unpaired → $\mu = \sqrt{3} = 1.73$ B.M.
• $Mn^{2+}$ ($d^5$, high spin): 5 unpaired → $\mu = \sqrt{35} = 5.92$ B.M. (maximum)

Catalytic Activity: Transition metals/compounds excellent catalysts due to:

• Multiple oxidation states (alternate paths)
• Surface adsorption (heterogeneous catalysis)
• Complex formation

Examples:

• Fe in Haber process
• V₂O₅ in Contact process
• Ni in hydrogenation of vegetable oils
• Pt-Rh in Ostwald process

Complex Formation: Empty $d$-orbitals + small size + variable ONs allow coordination compound formation.

Examples: $[Fe(CN)_6]^{4-}$, $[Cu(NH_3)_4]^{2+}$, $[CoCl_4]^{2-}$.

Alloy Formation: Similar atomic sizes (within $\sim 10\%$) → form alloys easily.

• Brass: Cu + Zn
• Bronze: Cu + Sn
• Steel: Fe + C (sometimes Ni, Cr)
• Stainless steel: Fe + Cr + Ni

Interstitial Compounds: Small atoms (H, C, N, B) occupy interstitial sites in transition metal lattice. Hard, high MP, chemically inert. Examples: TiC, $Fe_3C$ (cementite), TiN, VH.

f-Block elements: Have partially filled $4f$ (lanthanides) or $5f$ (actinides) orbitals.

Lanthanides (Ce to Lu): Elements $58-71$. Filling $4f$ orbital.

General Configuration: $[Xe]\,4f^{1-14}\,5d^{0-1}\,6s^2$

General Properties:

• Silvery-white metals; soft
• Good conductors
• $+3$ is most common oxidation state; some show $+2$ (Eu, Yb) or $+4$ (Ce, Tb)
• Colored ions (due to $f$-$f$ transitions, sometimes); some colorless ($La^{3+}$, $Lu^{3+}$)
• Most $Ln^{3+}$ are paramagnetic

Lanthanide Contraction: Steady decrease in atomic and ionic radius across the lanthanide series (Ce to Lu).

Cause: Poor shielding by 4f electrons (which are diffuse and inner-located). Effective nuclear charge increases steadily, pulling electrons closer.

Consequences:

1. Similar atomic radii of 4d (Zr, Hf etc.) and 5d (Hf, Mo etc.) elements — separation difficult
2. Difficulty in separation of lanthanides — chemically very similar
3. Density increase down the series
4. Increase in basicity of $Ln(OH)_3$ from $La(OH)_3$ to $Lu(OH)_3$? Actually, decreases — smaller cation → less basic hydroxide
5. Increase in ionization enthalpy along the series

Important point: $Zr$ and $Hf$ have almost identical radii ($160$ pm) because of lanthanide contraction; chemically very similar — costly to separate.

Use of lanthanides:

• Misch metal ($Ce + La + Nd$): in cigarette lighters, tracer bullets
• $CeO_2$: catalyst, polishing agent
• $La_2O_3$: optical glasses
• Magnets ($Nd_2Fe_{14}B$): hard disks, motors
• Lasers ($Nd^{3+}$ in YAG laser)

Actinides (Th to Lr): Elements $90-103$. Filling $5f$ orbital.

General Configuration: $[Rn]\,5f^{1-14}\,6d^{0-1}\,7s^2$

Properties:

• All radioactive
• Th, U found in nature; rest are synthetic
• Show many oxidation states (more than lanthanides): U shows $+3, +4, +5, +6$
• Form complexes more readily than lanthanides

Difference from Lanthanides:

Important Compounds:

Potassium Permanganate ($KMnO_4$):

Preparation:

• $2MnO_2 + 4KOH + O_2 \to 2K_2MnO_4 + 2H_2O$ (Mn $+4 \to +6$, manganate)
• $3MnO_4^{2-} + 4H^+ \to 2MnO_4^- + MnO_2 + 2H_2O$ (disproportionation in acidic)
• Or electrolytic oxidation of $MnO_4^{2-}$

Properties:

• Dark purple crystals
• Strong oxidizing agent in acidic, neutral, and basic media
• Color due to charge transfer

Reactions (Acidic medium):

• $MnO_4^- + 8H^+ + 5e^- \to Mn^{2+} + 4H_2O$ ($n$-factor 5; $E = M/5 = 31.6$)
• Oxidizes Fe²⁺: $5Fe^{2+} + MnO_4^- + 8H^+ \to 5Fe^{3+} + Mn^{2+} + 4H_2O$
• Oxidizes $C_2O_4^{2-}$: $5C_2O_4^{2-} + 2MnO_4^- + 16H^+ \to 10CO_2 + 2Mn^{2+} + 8H_2O$
• Oxidizes $I^-$ to $I_2$; $H_2S$ to $S$; $SO_2$ to $SO_4^{2-}$

Reactions (Basic/Neutral medium):

• $MnO_4^- + 2H_2O + 3e^- \to MnO_2 + 4OH^-$ ($n$-factor 3)

Reactions (Strongly basic, Strong reducing agent):

• $MnO_4^- + e^- \to MnO_4^{2-}$ (ON: $+7 \to +6$; $n$-factor 1)

Uses: Oxidizing agent in lab; volumetric analysis (self-indicator); bleaching wool, silk; disinfectant.

Potassium Dichromate ($K_2Cr_2O_7$):

Preparation:

• From chromite ore $FeCr_2O_4$:

$4FeCr_2O_4 + 16NaOH + 7O_2 \to 8Na_2CrO_4 + 2Fe_2O_3 + 8H_2O$

• $2Na_2CrO_4 + H_2SO_4 \to Na_2Cr_2O_7 + Na_2SO_4 + H_2O$
• $Na_2Cr_2O_7 + 2KCl \to K_2Cr_2O_7 + 2NaCl$ (less soluble)

Properties:

• Orange-red crystals
• Stable in air
• Strong oxidizing agent in acidic medium

$CrO_4^{2-}$ vs $Cr_2O_7^{2-}$:

• In acidic: $2CrO_4^{2-} + 2H^+ \to Cr_2O_7^{2-} + H_2O$ (orange)
• In basic: $Cr_2O_7^{2-} + 2OH^- \to 2CrO_4^{2-} + H_2O$ (yellow)

Reactions:

• $Cr_2O_7^{2-} + 14H^+ + 6e^- \to 2Cr^{3+} + 7H_2O$ ($n$-factor 6; $E = M/6 = 49$)
• Oxidizes Fe²⁺: $6Fe^{2+} + Cr_2O_7^{2-} + 14H^+ \to 6Fe^{3+} + 2Cr^{3+} + 7H_2O$
• Oxidizes $I^-$ to $I_2$
• Oxidizes $H_2S$ to $S$

Chromyl Chloride Test: $K_2Cr_2O_7 + 4NaCl + 6H_2SO_4 \to 2CrO_2Cl_2$ (orange-red vapors) $+ 4NaHSO_4 + 2KHSO_4 + 3H_2O$. Test for $Cl^-$.

Uses: Volumetric analysis (vs Fe²⁺), tanning of leather, dyes, photography.

Comparison of $KMnO_4$ vs $K_2Cr_2O_7$: