D-Block Elements | CBSE - Wyatt's Notes

sources:

• text: Standard textbook reference

D-block elements (transition metals) have partially filled d-orbitals. They exhibit variable oxidation states, colored compounds, catalytic properties, and complex formation.

• Transition metals: Sc to Zn (Groups 3-12), filling 3d orbitals
• Variable oxidation states: due to small energy difference between (n-1)d and ns orbitals
• Colored compounds: d-d transitions absorb visible light
• Catalytic properties: ability to adopt multiple oxidation states (MnO$_2$, V$_2$O$_5$, Fe)
• Complex formation: transition metals form coordination compounds
• Paramagnetism: unpaired electrons in d-orbitals
• Interstitial compounds: small atoms (H, C, N) trapped in metal lattice
• Alloy formation: similar atomic radii allow substitution

Problem: Write the electron configurations and predict the number of unpaired electrons for Ti$^{3+}$, Fe$^{2+}$, and Cu$^{2+}$.

Solution:

Ti$^{3+}$ (Z = 22): Ti: $[\text{Ar}] 3d^2 4s^2$ Ti$^{3+}$: $[\text{Ar}] 3d^1$ Unpaired electrons: 1

Fe$^{2+}$ (Z = 26): Fe: $[\text{Ar}] 3d^6 4s^2$ Fe$^{2+}$: $[\text{Ar}] 3d^6$ Unpaired electrons: 4 (using Hund’s rule: $\uparrow\downarrow, \uparrow, \uparrow, \uparrow, \uparrow$)

Cu$^{2+}$ (Z = 29): Cu: $[\text{Ar}] 3d^{10} 4s^1$ (exception) Cu$^{2+}$: $[\text{Ar}] 3d^9$ Unpaired electrons: 1

Common mistake: Writing Cu as $[\text{Ar}] 3d^9 4s^2$. Copper has the anomalous configuration $[\text{Ar}] 3d^{10} 4s^1$ (fully filled d is more stable).

Problem: Explain why Ti$^{3+}$ compounds are purple while Cu$^{2+}$ compounds are blue.

Solution:

Ti$^{3+}$: Configuration $3d^1$. One d electron can absorb visible light and jump to a higher d orbital. The absorbed wavelength corresponds to yellow-green, so the transmitted light appears purple (complementary color).

Cu$^{2+}$: Configuration $3d^9$. Nine d electrons with one vacancy. The d-d transition absorbs in the red-orange region, so the transmitted light appears blue.

The color depends on:

1. The number of d electrons (determines which transitions are possible)
2. The ligand field strength (affects the energy gap $\Delta$)
3. The geometry (octahedral, tetrahedral)

Common mistake: Thinking the color comes from s or p electrons. Transition metal colors arise from d-d transitions.

Problem: Explain the catalytic role of MnO$_2$ in the decomposition of $\text{H}_2\text{O}_2$.

Solution:

MnO$_2$ acts as a heterogeneous catalyst. The mechanism involves:

Step 1: $\text{MnO}_2 + \text{H}_2\text{O}_2 \rightarrow \text{MnO}_3 + \text{H}_2\text{O}$ (Mn$^{4+}$ oxidized to Mn$^{6+}$)

Step 2: $\text{MnO}_3 + \text{H}_2\text{O}_2 \rightarrow \text{MnO}_2 + \text{H}_2\text{O} + \text{O}_2$ (Mn$^{6+}$ reduced back to Mn$^{4+}$)

The catalyst cycles between oxidation states, providing an alternative pathway with lower activation energy.

Common mistake: Thinking the catalyst is consumed. MnO$_2$ is regenerated at the end of the reaction.

1. Write the electron configuration of Cr$^{3+}$ and predict its magnetic moment.
2. Why are transition metal oxides amphoteric?
3. Explain why Fe$^{2+}$ is a reducing agent while Fe$^{3+}$ is an oxidizing agent.

Transition metals are essential for industrial catalysis (Haber process, catalytic converters), biological systems (hemoglobin, chlorophyll), and technology (batteries, electronics, magnets).

Colorful metals with multiple personalities: Transition metals are like actors who can play many roles — iron can be Fe²⁺ or Fe³⁺, copper can be Cu⁺ or Cu²⁺, and manganese can exist in seven different oxidation states. This versatility comes from their partially filled d-orbitals, which are close in energy to the s-orbitals. When light hits a transition metal compound, d-electrons absorb specific wavelengths and jump to higher energy levels — the remaining light gives the compound its characteristic color, like how stained glass filters sunlight into colors.

Why it matters: Transition metals are the workhorses of industry and biology. Iron carries oxygen in hemoglobin, copper conducts electricity in wires, titanium makes aircraft strong and light, and platinum catalyzes chemical reactions. Understanding d-block chemistry means understanding the elements that build and power our world.

The key insight: The anomalous electron configurations of Cr ([Ar]3d⁵4s¹) and Cu ([Ar]3d¹⁰4s¹) occur because half-filled and fully-filled d-orbitals are extra stable — nature always finds the lowest energy arrangement.

• Electron configurations: remember anomalies (Cr, Cu)
• Color: d-d transitions, depends on oxidation state and ligand
• Catalysis: variable oxidation states enable redox cycling
• Complex formation: CN$^-$, NH$_3$, H$_2$O as common ligands
• Magnetic moment: $\mu = \sqrt{n(n+2)}$ BM where $n$ = unpaired electrons

Writing copper as [Ar] 3d9 4s2. Copper has the anomalous configuration [Ar] 3d10 4s1 because a fully filled d-subshell is more stable than a partially filled one. Similarly, chromium is [Ar] 3d5 4s1, not [Ar] 3d4 4s2. These exceptions arise from the extra stability of half-filled and fully filled d-orbitals.

Assuming the catalyst is consumed in a reaction. Catalysts provide an alternative reaction pathway with lower activation energy but are regenerated at the end. MnO2 in the decomposition of H2O2 cycles between Mn4+ and Mn6+ oxidation states but is recovered unchanged. A catalyst is not a reactant.

Confusing the colour origin in transition metal compounds. The characteristic colours of transition metal ions arise from d-d electronic transitions, not from s or p electrons. When white light passes through a solution, certain wavelengths are absorbed to promote d electrons to higher energy levels, and the transmitted complementary colour is observed.

• Coordination Compounds: Crystal field theory explains d-orbital splitting in coordination complexes — connecting d-block properties to coordination chemistry.
• F-Block Elements: Lanthanoid contraction affects Period 6 d-block elements (Zr ≈ Hf), linking f-block trends to d-block chemistry.
• Electrochemistry: Variable oxidation states enable redox cycling in electrochemical cells — connecting d-block properties to batteries and electrolysis.
• Surface Chemistry: Transition metals are heterogeneous catalysts — their surface properties and d-electrons enable catalytic activity.