JEE Main d and f Block Elements 2027 — Electronic Configuration, Properties and 40 Practice MCQs
The d and f block elements chapter is one of the highest-scoring topics in JEE Main 2027 Chemistry. Every JEE Main paper carries 2–4 questions from transition metals and inner transition metals. This comprehensive guide covers electronic configurations, physical and chemical properties, important compounds like KMnO₄ and K₂Cr₂O₇, lanthanide contraction, and magnetic properties — everything you need to score full marks on this chapter.
What Are d-Block Elements?
d-Block elements are those in which the last electron enters the d-orbital of the penultimate shell. They occupy Groups 3–12 in the periodic table and are commonly called transition metals. The general electronic configuration is (n-1)d¹⁻¹⁰ ns⁰⁻².
These elements bridge the highly electropositive s-block metals and the electronegative p-block elements. They are characterized by variable oxidation states, formation of coloured compounds, catalytic activity, and complex formation — all JEE-favourite topics.
Electronic Configurations — Key Anomalies
The most important JEE question-generator in this chapter is the anomalous electronic configuration of Cr and Cu:
- Chromium (Cr, Z=24): Expected [Ar] 3d⁴ 4s² → Actual [Ar] 3d⁵ 4s¹ (half-filled d-orbital is extra stable)
- Copper (Cu, Z=29): Expected [Ar] 3d⁹ 4s² → Actual [Ar] 3d¹⁰ 4s¹ (fully filled d-orbital gives extra stability)
Rule: Half-filled (d⁵) and fully filled (d¹⁰) d-orbitals are extra stable due to symmetrical distribution and maximum exchange energy.
Physical Properties of d-Block Elements
| Property | Trend / Value | Reason | JEE Fact |
|---|---|---|---|
| Melting/Boiling Point | High; W has highest (3422°C) | Strong metallic bonding via d-electrons | Hg is liquid at room temp — lowest MP |
| Atomic Radius | Decreases slightly across period | Increasing nuclear charge, d-electrons shield poorly | Zr ≈ Hf radius (lanthanide contraction effect) |
| Ionization Enthalpy | Higher than s-block; increases irregularly | Effective nuclear charge increase | Zn has highest in 1st row (d¹⁰ stable) |
| Density | High density metals | Small atomic size + heavy mass | Os is densest element known |
| Electrical Conductivity | High (Cu, Ag best conductors) | Delocalized d and s electrons | Silver = best conductor of electricity |
| Magnetic Properties | Para or ferromagnetic | Unpaired d-electrons | Mn²⁺ has 5 unpaired e⁻ → μ = 5.92 BM |
Variable Oxidation States
Transition metals show multiple oxidation states because both (n-1)d and ns electrons have similar energies and can be used in bonding.
- Mn shows maximum variety: +2, +3, +4, +5, +6, +7
- Fe commonly shows +2 and +3
- Cu shows +1 and +2 (Cu²⁺ more stable in aqueous solution)
- Highest oxidation state is seen in fluorides and oxides (e.g., MnO₄⁻ has Mn in +7)
Important Compounds: KMnO₄ and K₂Cr₂O₇
Potassium Permanganate (KMnO₄)
KMnO₄ is a powerful oxidising agent. Mn is in +7 oxidation state.
- In acidic medium (H₂SO₄): MnO₄⁻ → Mn²⁺ (gain of 5e⁻ per Mn; purple → colourless)
- In neutral/faintly alkaline medium: MnO₄⁻ → MnO₂ (gain of 3e⁻; brown precipitate)
- In strongly alkaline medium: MnO₄⁻ → MnO₄²⁻ (gain of 1e⁻; green manganate)
Potassium Dichromate (K₂Cr₂O₇)
K₂Cr₂O₇ acts as an oxidising agent in acidic medium. Cr is in +6 state.
- Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (orange → green)
- Used in volumetric analysis to estimate Fe²⁺, Sn²⁺, I⁻
Colour of Transition Metal Compounds
Transition metal compounds are coloured because of d-d transitions: electrons absorb visible light to jump from lower-energy d-orbitals to higher-energy ones. The complementary colour is observed.
| Ion | Configuration | Colour |
|---|---|---|
| Cu²⁺ (CuSO₄·5H₂O) | [Ar] 3d⁹ | Blue |
| Fe³⁺ | [Ar] 3d⁵ | Yellow/Brown |
| Cr³⁺ | [Ar] 3d³ | Violet/Green |
| Mn²⁺ | [Ar] 3d⁵ | Pale pink (faint) |
| Zn²⁺ (d¹⁰) | [Ar] 3d¹⁰ | Colourless (no d-d transition possible) |
f-Block Elements: Lanthanides and Actinides
f-Block elements are inner transition metals where the last electron enters the f-orbital of the ante-penultimate shell.
- Lanthanides (4f series): Ce (58) to Lu (71) — total 14 elements
- Actinides (5f series): Th (90) to Lr (103) — total 14 elements
Lanthanide Contraction
As we move across lanthanide series, atomic and ionic radii decrease gradually. This is called lanthanide contraction — caused by poor shielding by f-electrons combined with increasing nuclear charge.
Consequences of Lanthanide Contraction:
- Zr and Hf have nearly identical radii (160 pm each) → difficult to separate
- Similar properties of 4d and 5d transition metals in same group
- Explains why Au and Pt are noble (relativistic effects + lanthanide contraction)
Magnetic Moment Formula
The spin-only magnetic moment formula: μ = √(n(n+2)) BM
where n = number of unpaired electrons
| Unpaired e⁻ (n) | μ (BM) | Example |
|---|---|---|
| 1 | 1.73 | Cu²⁺ (3d⁹) |
| 2 | 2.83 | Ni²⁺ (3d⁸) |
| 3 | 3.87 | Co²⁺ (3d⁷) |
| 4 | 4.90 | Fe²⁺ (3d⁶) |
| 5 | 5.92 | Mn²⁺ / Fe³⁺ (3d⁵) |
Catalytic Properties
Transition metals are excellent catalysts because:
- Variable oxidation states allow them to form intermediates
- Large surface area (particularly in finely divided form)
Examples: Fe in Haber process (N₂ + H₂ → NH₃), V₂O₅ in Contact process (SO₂ → SO₃), Ni in hydrogenation of oils, Pt in Ostwald process.
10 High-Yield JEE MCQs — Practice Now
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Frequently Asked Questions (FAQ)
- Q: Why do transition metals show variable oxidation states?
- Because both (n-1)d and ns electrons have similar energies and can participate in bonding. The small energy gap allows different numbers of electrons to be involved in bond formation, giving multiple oxidation states.
- Q: What is lanthanide contraction and why is it important for JEE?
- Lanthanide contraction is the steady decrease in atomic radii across the lanthanide series due to poor f-electron shielding. Key JEE consequence: Zr and Hf have nearly identical radii (~160 pm each) making them hard to separate — a classic exam question.
- Q: Why does Cu have [Ar] 3d¹⁰ 4s¹ configuration?
- A fully filled d-orbital (3d¹⁰) provides extra stability due to symmetrical electron distribution and maximum exchange energy, overriding normal aufbau filling order.
- Q: How many JEE questions come from d-block?
- JEE Main typically has 2–4 questions. High-yield topics: Cr and Cu configurations, KMnO₄ reactions in different media, colour of ions, lanthanide contraction, and magnetic moment calculations.
- Q: What is the magnetic moment of Fe³⁺?
- Fe³⁺ is [Ar] 3d⁵ with 5 unpaired electrons. μ = √(5×7) = √35 ≈ 5.92 BM — same as Mn²⁺. This is the maximum spin-only magnetic moment for first-row transition metal ions.
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