The d- and f-Block Elements: A Comprehensive Study

Overview of d-Block and f-Block Elements

• The d-Block: Groups 3-12 where d-orbitals are filled. These are the transition metals.
• The f-Block: Elements filling 4f (Lanthanoids) and 5f (Actinoids) orbitals. These are the inner transition metals.
• Transition Metal Definition: According to IUPAC, metals with an incomplete d-subshell in a neutral atom or common ions.
• Group 12 Exceptions: Zinc ($Zn$), Cadmium ($Cd$), and Mercury ($Hg$) are not technically transition metals because they have full $d^{10}$ configurations in ground and common oxidation states.
• Series: Four series exist: 3d ($Sc$ to $Zn$), 4d ($Y$ to $Cd$), 5d ($La$ and $Hf$ to $Hg$), and 6d ($Ac$ and $Rf$ to $Cn$).

Electronic Configuration

• General Formula: $(n-1)d^{1-10} ns^{1-2}$. (Exception: Palladium is $4d^{10} 5s^0$).
• Stability: Half-filled and completely filled orbitals are more stable.
• Chromium ($Cr$): Configured as $3d^5 4s^1$ instead of $3d^4 4s^2$.
• Copper ($Cu$): Configured as $3d^{10} 4s^1$ instead of $3d^9 4s^2$.

Physical and Atomic Properties

• Metallic Properties: High tensile strength, ductility, malleability, high thermal/electrical conductivity, and metallic lustre.
• Melting Points: Generally high due to the involvement of $(n-1)d$ and $ns$ electrons in interatomic metallic bonding. Maxima occur near the middle of each series ($d^5$).
• Atomic/Ionic Radii: Gradual decrease across a series due to increasing nuclear charge and poor shielding by d-electrons.
• Lanthanoid Contraction: The regular decrease in size across the 4f series. This causes the radii of the second (4d) and third (5d) series to be nearly identical (e.g., $Zr = 160\,pm$, $Hf = 159\,pm$).
• Ionisation Enthalpy: Increases along the series but less steeply than in non-transition elements. First ionisation energy increases only slightly because 3d electrons shield 4s electrons.

Chemical Characteristics of d-Block Elements

• Oxidation States: Exhibit a wide variety because the $(n-1)d$ and $ns$ orbitals have similar energy. Manganese ($Mn$) shows the most states (+2 to +7).
• Magnetic Properties: Primarily paramagnetic due to unpaired electrons.
  • Spin-only formula: $\mu = \sqrt{n(n+2)}$ where $n$ is the number of unpaired electrons and $\mu$ is the magnetic moment in Bohr magnetons ($BM$).
• Coloured Ions: Formed when d-electrons are excited to higher energy d-orbitals using frequencies of light in the visible region.
• Complex Formation: Transition metals form complexes due to small ion size, high ionic charge, and available d-orbitals.
• Catalytic Activity: Attributed to multiple oxidation states and the ability to form complexes (e.g., $V_2O_5$ in the Contact Process, $Fe$ in the Haber Process).
• Interstitial Compounds: Formed when small atoms ($H$, $C$, $N$) are trapped in metal lattices. Characteristics: High melting points, extreme hardness, chemical inertness.

Important Compounds: $K_2Cr_2O_7$ and $KMnO_4$

• Potassium Dichromate ($K_2Cr_2O_7$):
  • Preparation: Obtained from chromite ore ($FeCr_2O_4$) via fusion with sodium carbonate, followed by acidification to produce sodium dichromate ($Na_2Cr_2O_7$) and reaction with $KCl$.
  • Equilibrium: Chromate ($CrO_4^{2-}$) and dichromate ($Cr_2O_7^{2-}$) are interconvertible based on pH ($CrO_4^{2-}$ in alkaline, $Cr_2O_7^{2-}$ in acidic).
  • Oxidation: Strong oxidant in acidic medium: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$.
• Potassium Permanganate ($KMnO_4$):
  • Preparation: Fusion of $MnO_2$ with hydroxide and oxidant ($KNO_3$) produces green $K_2MnO_4$, which is then oxidized electrolytically.
  • Oxidation: In acidic solution: $MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$. It oxidizes iodides to iodine, $Fe^{2+}$ to $Fe^{3+}$, and oxalates to $CO_2$.

The Inner Transition Elements (f-Block)

• Lanthanoids: Filling 4f orbitals. Predominant stable oxidation state is +3. Silvery soft metals.
• Actinoids: Filling 5f orbitals. All are radioactive.
  • Oxidation States: More varied than lanthanoids because 5f, 6d, and 7s levels have comparable energies (Th up to +4, Pa up to +5, U up to +6, Np up to +7).
  • Actinoid Contraction: Greater decrease in size per element than lanthanoid contraction due to even poorer shielding by 5f electrons.

Questions & Discussion

• Q: Why is scandium ($Z = 21$) a transition element while zinc ($Z = 30$) is not?
• A: Scandium has an incompletely filled 3d orbital ($3d^1$), whereas zinc has a completely filled 3d subshell ($3d^{10}$) in both its ground and oxidized states.
• Q: Why do transition elements have high enthalpies of atomisation?
• A: Due to the large number of unpaired electrons participating in strong interatomic bonding.
• Q: Why is the $E^\circ(M^{2+}/M)$ value for copper positive ($+0.34\,V$)?
• A: High enthalpy of atomisation and low hydration enthalpy mean the energy required to transform solid $Cu$ to $Cu^{2+}(aq)$ is not balanced.
• Q: Which 3d transition metal exhibits the largest number of oxidation states?
• A: Manganese, because it has the maximum number of unpaired electrons available for bonding.
• Q: Why is actinoid contraction greater than lanthanoid contraction?
• A: The 5f electrons provide poorer shielding from nuclear charge compared to 4f electrons.
• Q: Why can silver ($Z = 47$) be called a transition element if it has a $4d^{10}$ ground state?
• A: It can exhibit the +2 oxidation state where it has an incompletely filled d-subshell ($4d^9$).