d- and f-Block Elements — chemistry Class 12 Notes (CBSE & HBSE)
Free NCERT chemistry notes for d- and f-Block Elements (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — d- and f-Block Elements (CBSE & HBSE)
Transition elements: electronic configuration, properties (variable oxidation states, colored compounds, magnetic behavior, catalytic activity), important compounds KMnO4 and K2Cr2O7, lanthanoids and actinoids.
Transition Elements - Properties
d-Block Elements (Transition Metals)
Definition
Elements in which the last electron enters d-orbital. d-block: Groups 3-12. Transition metals: Elements with incomplete d-subshell in the element OR its ions. (Zn, Cd, Hg are d-block but NOT transition metals: they have d^10 in all states)
Electronic Configuration
(n-1)d^1-10 ns^0-2 Note: Cr: [Ar] 3d^5 4s^1 (half-filled d^5 more stable than d^4 4s^2) Cu: [Ar] 3d^10 4s^1 (fully filled d^10 more stable than d^9 4s^2)
Properties of Transition Metals
1. Variable Oxidation States: Small energy difference between (n-1)d and ns electrons; both can be lost.
| Metal | Common Oxidation States |
|---|---|
| Fe | +2, +3, +6 |
| Cu | +1, +2 |
| Mn | +2, +3, +4, +6, +7 |
| Cr | +2, +3, +6 |
| V | +2, +3, +4, +5 |
Most stable: Mn(+2), Fe(+2/+3), Cu(+2), Cr(+3)
2. Colored Compounds: Transition metal ions absorb visible light. Partially filled d-orbitals allow d-d transitions. Color is complementary to absorbed color.
| Ion | Color |
|---|---|
| Ti3+ | Purple |
| V3+ | Green |
| Cr3+ | Violet |
| Mn2+ | Pale pink |
| Fe3+ | Yellow-brown |
| Co2+ | Pink |
| Cu2+ | Blue |
| Ni2+ | Green |
Zn2+ (d^10) and Sc3+ (d^0) are colorless (no d-d transition possible).
3. Magnetic Behavior: Depends on number of unpaired d-electrons. More unpaired electrons = higher magnetic moment. Magnetic moment (BM) = sqrt(n(n+2)) where n = unpaired electrons. Mn2+ (d^5, 5 unpaired): mu = sqrt(35) = 5.92 BM (highest for first transition series)
4. Catalytic Activity: Transition metals and their compounds are excellent catalysts:
- Fe: Haber process (N2 + H2 → NH3)
- V2O5: Contact process (SO2 → SO3)
- Ni: Hydrogenation of fats
- Pt, Pd: Catalytic converters, hydrogenation
- MnO2: KClO3 decomposition
Reason: Variable oxidation states allow formation of intermediate complexes.
5. Complex Formation: Transition metals readily form complex compounds due to:
- Small atomic/ionic size
- High charge density
- Empty d-orbitals available for ligand coordination
6. Interstitial Compounds: Small atoms (H, B, C, N) occupy interstitial positions in metal lattice. Example: Steel (Fe + C), TiH2, TiC. Hardness, melting points, and conductivities usually higher.
7. Alloy Formation: Similar atomic sizes allow substitution in crystal lattice. Examples: Stainless steel (Fe+Cr+Ni), brass (Cu+Zn), bronze (Cu+Sn).
Important Compounds of Transition Metals
Important Compounds
Potassium Dichromate (K2Cr2O7)
Preparation: Step 1: Roast chromite ore (FeCr2O4) with Na2CO3 in air: 4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2
Step 2: Acidify sodium chromate solution: 2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
Step 3: Treat with KCl: Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
Structure: Cr2O7^2- (dichromate ion): Two CrO4 tetrahedra sharing one O atom. Cr is +6.
Properties:
- Orange crystals; strong oxidizing agent in acidic medium
- Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O (E0 = +1.33 V)
- In basic medium: 2CrO4^2- (chromate, yellow) ↔ Cr2O7^2- + H2O (pH dependent)
Reactions as Oxidizing Agent:
- Oxidizes Fe2+ → Fe3+: K2Cr2O7 + 6FeSO4 + 7H2SO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O
- Oxidizes I- → I2
- Oxidizes H2S → S
- Oxidizes SO2 → SO3
Uses: Tanning of leather, dyeing, oxidizing agent in organic synthesis.
Potassium Permanganate (KMnO4)
Preparation: Step 1: Fuse MnO2 with KOH in air: 2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O
Step 2: Oxidize K2MnO4 (green, manganate) to KMnO4 (purple, permanganate): 3K2MnO4 + 2CO2 → 2KMnO4 + MnO2 + 2K2CO3 or electrolytic oxidation.
Structure: MnO4- (permanganate ion): Tetrahedral; Mn is +7.
Properties:
- Purple/violet crystals (most soluble salt of Mn)
- Very strong oxidizing agent
- Action depends on pH:
In ACIDIC medium: MnO4- → Mn2+ (Mn reduced from +7 to +2) MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
In NEUTRAL/WEAKLY ALKALINE: MnO4- → MnO2 (brown ppt; Mn: +4) MnO4- + 2H2O + 3e- → MnO2 + 4OH-
In STRONGLY ALKALINE: MnO4- → MnO4^2- (manganate; Mn: +6) MnO4- + e- → MnO4^2-
Important Reactions of KMnO4 (acidic):
- Oxidizes Fe2+ → Fe3+: MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O
- Oxidizes I- → I2: 2MnO4- + 10I- + 16H+ → 2Mn2+ + 5I2 + 8H2O
- Oxidizes Oxalic acid: 2MnO4- + 5H2C2O4 + 6H+ → 2Mn2+ + 10CO2 + 8H2O
- Decolorizes acidified KMnO4 with H2O2: 2MnO4- + 5H2O2 + 6H+ → 2Mn2+ + 5O2 + 8H2O
Uses: Volumetric analysis (titrations), disinfectant (Condy's fluid), bleaching.
f-Block Elements - Lanthanoids and Actinoids
f-Block Elements
Lanthanoids (Lanthanides)
Elements: La (57) to Lu (71) (14 elements after La in Period 6) Electronic configuration: [Xe] 4f^1-14 5d^0-1 6s^2
| Property | Lanthanoids |
|---|---|
| Oxidation states | +3 most common; +2 (Eu, Sm, Yb); +4 (Ce, Tb, Pr) |
| Color | Many are colored (f-f transitions) |
| Magnetic behavior | Paramagnetic (unpaired f electrons) |
| Atomic/ionic radii | Lanthanoid contraction |
Lanthanoid Contraction: Progressive decrease in ionic radii (La3+ to Lu3+) across lanthanoid series.
Reason: As atomic number increases, nuclear charge increases by 1 each time, adding an electron to 4f orbital. The 4f electrons are poor shielders of nuclear charge (f orbitals have poor shielding). The increased nuclear charge contracts the electron cloud more than it expands due to extra electron. Net effect: steady decrease in size.
Consequences of Lanthanoid Contraction:
- Post-lanthanoid transition metals (Hf, Ta, W...) have nearly same atomic radii as 5d metals (Zr, Nb, Mo...) → difficult to separate
- 5d transition metals more similar to 4d (Period 5) metals than expected
- La and Lu are very similar in properties
Chemical Properties of Lanthanoids:
- React with water (slowly), dissolve in dilute acids
- Form stable +3 compounds (oxides, hydroxides, halides)
- Lanthanoids + H2O2 → L2O3 + H2O
- Hard to separate (similar properties due to lanthanoid contraction)
Actinoids (Actinides)
Elements: Ac (89) to Lr (103) (14 elements after Ac in Period 7) Electronic configuration: [Rn] 5f^1-14 6d^0-1 7s^2
| Property | Difference from Lanthanoids |
|---|---|
| Oxidation states | +2 to +7 (more variable than lanthanoids) |
| Radioactivity | All radioactive (no stable isotopes for heavy actinoids) |
| 5f vs 4f | 5f better shielded; actinoid contraction similar to lanthanoid |
| Common states | U shows +3 to +6; Np +3 to +7 |
Important Actinoids:
- U-235: Nuclear fission (nuclear reactor and bombs)
- Pu-239: Nuclear weapons
- Th-232: Nuclear fuel (India has large Th reserves)
- Am-241: Smoke detectors
Actinoid Contraction: Similar to lanthanoid contraction but 5f electrons shield even less than 4f. Steady decrease in atomic/ionic radii across actinoid series.
Lanthanoids vs Actinoids:
| Property | Lanthanoids | Actinoids |
|---|---|---|
| Filling orbital | 4f | 5f |
| Radioactivity | Not radioactive | All radioactive |
| Oxidation states | Mostly +3 | +2 to +7 |
| Nuclear stability | Stable nuclei | All unstable |
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