d-Block (Transition Metals) & f-Block

Q1. (3 marks)

• General configuration: $(n-1)d^{1-10}\,ns^{0-2}$ (1)
• Zn, Cd, Hg have completely filled $d^{10}$ configuration in both atomic and common ionic ($M^{2+}$) states. (1)
• Transition elements are defined as having partially filled d-orbitals in atom or ion; since Zn/Cd/Hg have full $d^{10}$, they are not typical transition elements. (1)

Q2. (3 marks, 1 each)

• (a) $\text{Cr}$: $[\text{Ar}]\,3d^5\,4s^1$ (half-filled stability). (1)
• (b) $\text{Cu}^{2+}$: $[\text{Ar}]\,3d^9$. (1)
• (c) $\text{Fe}^{3+}$: $[\text{Ar}]\,3d^5$. (1)

Q3. (3 marks, 1 each — any three)

• Small energy difference between $(n-1)d$ and $ns$ orbitals, so both can participate in bonding.
• $(n-1)d$ and $ns$ electrons have comparable energies → variable numbers of electrons lost.
• Extra stability of half-filled ($d^5$) and fully-filled ($d^{10}$) configurations in certain states. (Award 1 mark each; max 3.)

Q4. (4 marks)

• Definition: The steady decrease in atomic and ionic radii of the lanthanide elements with increasing atomic number (from La to Lu). (2)
• Cause: Poor shielding of the nuclear charge by 4f electrons, so effective nuclear charge increases across the series, pulling electrons closer. (1)
• Consequence (any one): Zr and Hf (and Nb/Ta) have almost identical radii; difficulty in separating lanthanides; increase in density down the group. (1)

Q5. (6 marks, 2 each) Spin-only: $\mu=\sqrt{n(n+2)}$ BM.

• (a) $\text{Ti}^{3+}$: $3d^1$, $n=1$ → $\mu=\sqrt{1\cdot3}=\sqrt3=1.73$ BM. (2)
• (b) $\text{Mn}^{2+}$: $3d^5$, $n=5$ → $\mu=\sqrt{5\cdot7}=\sqrt{35}=5.92$ BM. (2)
• (c) $\text{Ni}^{2+}$: $3d^8$, $n=2$ → $\mu=\sqrt{2\cdot4}=\sqrt8=2.83$ BM. (2) (1 mark for correct n, 1 mark for value.)

Q6. (4 marks)

• Transition-metal ions have partially filled d-orbitals. (1)
• In a complex/crystal field, d-orbitals split into groups of slightly different energy. (1)
• An electron absorbs visible light and undergoes a d-d transition; the complementary colour is transmitted/observed. (1)
• $\text{Sc}^{3+}$ ($3d^0$) and $\text{Zn}^{2+}$ ($3d^{10}$) have empty/completely filled d-orbitals → no d-d transition possible → colourless. (1)

Q7. (4 marks, 1 each)

• (a) Contact process: $\text{V}_2\text{O}_5$. (1)
• (b) Haber process: Finely divided Fe (with Mo promoter). (1)
• (c) Hydrogenation of oils: Ni (Raney nickel). (1)
• (d) Pt (or Pt–Rh gauze in Ostwald process / Pt in catalytic converters). (1)

Q8. (5 marks)

• (a) $\text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$ (2)
• (b) $2\text{MnO}_4^- + 5\text{C}_2\text{O}_4^{2-} + 16\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 10\text{CO}_2 + 8\text{H}_2\text{O}$ (3) (1 mark correct species, 1 mark charge/electron balance, 1 mark atom balance.)

Q9. (4 marks)

• (a) Oxidation state of Cr in $\text{Cr}_2\text{O}_7^{2-}$: $+6$. (1)
• (b) $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \rightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$ (3) (1 mark Cr reduction, 1 mark Fe oxidation, 1 mark full balance.)

Q10. (4 marks, 1 each — any four)

(Award 1 mark per correct comparison, max 4.)

[
  {"claim":"Ti3+ (d1) spin-only moment = sqrt(3) ≈ 1.73 BM","code":"n=1; mu=sqrt(n*(n+2)); result = abs(float(mu)-1.7320508)<1e-4"},
  {"claim":"Mn2+ (d5) spin-only moment = sqrt(35) ≈ 5.92 BM","code":"n=5; mu=sqrt(n*(n+2)); result = abs(float(mu)-5.9160798)<1e-4"},
  {"claim":"Ni2+ (d8) spin-only moment = sqrt(8) ≈ 2.83 BM","code":"n=2; mu=sqrt(n*(n+2)); result = abs(float(mu)-2.8284271)<1e-4"},
  {"claim":"MnO4- + C2O4^2- acidic reaction: electron balance 2 MnO4 (5e each) = 5 C2O4 (2e each)","code":"result = (2*5)==(5*2)"},
  {"claim":"Cr2O7^2- oxidizes 6 Fe2+ (6 electrons transferred)","code":"cr_electrons=2*3; fe_electrons=6*1; result = cr_electrons==fe_electrons"}
]