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Inorganic Chemistry XII · CBSE 2025–26

5-Mark Concepts
Inorganic Chemistry

d & f Block Elements · Coordination Compounds — priority guide compiled from PYQs and top question banks.

2
Chapters
14
Total marks
4
Key 5-mark concepts
2
Always-asked topics

Overview

🎯 These topics appear in EVERY or NEARLY EVERY paper — prepare these first:

  1. Coordination Compounds — VBT & CFT — geometry, hybridisation, crystal field splitting diagrams  (7 marks, very regular in every paper)
  2. d & f Block — Lanthanoid Contraction — cause and three consequences  (4-5 marks, appears almost every year)
7
Coordination Compounds marks
7
d & f Block marks
🔬 d & f Block Elements
7 marks
  • Variable oxidation states — electronic configuration basis
    Compare with main group; explain why d-block shows variable OS
  • Lanthanoid contraction — cause, consequences, anomalous properties
    5-mark: explain contraction → list 3 consequences
  • KMnO₄ preparation and properties — acidic medium reactions
    Oxidising action in acid/neutral/alkaline medium
🔗 Coordination Compounds
7 marks
  • Werner's Theory + IUPAC nomenclature — complex ion naming rules
    Name → structure → charge on central metal
  • VBT — hybridisation, geometry, magnetic character
    Inner orbital vs outer orbital complex; show d-orbital diagrams
  • Crystal Field Theory — Δ splitting, high-spin vs low-spin
    Draw d-orbital splitting diagram; calculate CFSE
  • Isomerism — structural (ionisation, linkage) and geometrical
    Draw cis/trans isomers; explain ionisation isomers with example

🔥 Priority List — Most Repeated 5-Mark Topics

#TopicChapterStarsTip
1 Coordination Compounds — VBT Hybridisation Coordination ★★★★★ Electronic config → show d orbital occupancy → hybridisation → geometry → magnetic
2 Crystal Field Theory — d-orbital Splitting Coordination ★★★★☆ Draw octahedral splitting: t₂g vs eg; strong field → low spin; weak field → high spin
3 Lanthanoid Contraction d&f Block ★★★☆☆ Poor shielding by 4f electrons → Z_eff increases → radius decreases → 3 consequences
4 KMnO₄ Reactions in Acid / Neutral / Alkaline d&f Block ★★☆☆☆ Acid: Mn²⁺ (colourless); neutral: MnO₂ (brown); alkaline: MnO₄²⁻ (green)

Concept Patterns — Fixed Templates

Coordination Compounds — VBT Hybridisation
5 marks Draw diagram + explain magnetic nature
Fixed Pattern
① Write electronic config of metal ion (e.g., Co³⁺: [Ar]3d⁶)
② Strong-field ligand → electrons pair up in 3d (inner orbital)
③ Hybridisation: d²sp³ (octahedral, inner) or sp³d² (octahedral, outer)
④ Show orbital diagram with arrows
⑤ Count unpaired electrons → magnetic moment = √(n(n+2)) BM
Common Variants
→ [Fe(CN)₆]³⁻ (strong field, d²sp³, low spin)
→ [FeF₆]³⁻ (weak field, sp³d², high spin)
→ [Ni(CO)₄] tetrahedral, sp³; [Ni(CN)₄]²⁻ square planar, dsp²
d & f Block — Lanthanoid Contraction
5 marks Definition + cause + 3 consequences
Fixed Pattern
① Define: regular decrease in atomic/ionic radii of La to Lu
② Cause: poor shielding by 4f electrons → increasing Z_eff → radius contracts
③ Consequence 1: Zr and Hf have nearly identical radii (difficult to separate)
④ Consequence 2: 2nd and 3rd row d-block elements (Mo/W, Nb/Ta) have similar properties
⑤ Consequence 3: Basicity of lanthanoid hydroxides decreases La(OH)₃ → Lu(OH)₃
Common Variants
→ "Why are 2nd and 3rd row transition metals so similar in properties?"
→ "Explain anomalous properties of Pt and Au compared to Ni and Cu"
→ "Why are lanthanoids difficult to separate from each other?"

Strategy — Inorganic Chemistry

🔗 Coordination Compounds

  • VBT diagram: always start with free metal ion config
  • CFT: memorise Δ for common ligands (CO > CN⁻ > NH₃ > H₂O > F⁻)
  • Magnetic moment formula: μ = √(n(n+2)) BM
  • IUPAC naming: ligands alphabetically before metal
  • Isomers: always draw structural diagrams, not just names
  • Inner orbital (d²sp³) = strong field; outer orbital (sp³d²) = weak field

🔬 d & f Block

  • Lanthanoid contraction: 3 consequences (Zr/Hf, Pt/Au, 2nd/3rd row similar)
  • KMnO₄: learn all 3 medium reactions with colours
  • Variable OS: due to comparable energies of 3d and 4s electrons
  • Cr²⁺ and Cu²⁺ are special — extra stability of half-filled/filled d
  • Magnetic properties: use unpaired electron count

Model Answer Cards — 5-Mark Q&A

Coordination
VBT — explain hybridisation, geometry and magnetic character of [Fe(CN)₆]³⁻
How it's asked
  1. "Using VBT, predict the hybridisation, geometry and magnetic nature of [Co(NH₃)₆]³⁺" [5]
  2. "Explain inner orbital and outer orbital complexes with one example each." [5]
  3. "Draw d-orbital diagram for [Fe(CN)₆]⁴⁻. Predict magnetic character." [5]
Show Model Answer 5 marks
1 mark
Electronic Config:
Fe³⁺: [Ar] 3d⁵ (free ion: 5 unpaired electrons)
CN⁻ is a strong field ligand → causes pairing of 3d electrons
2 marks
Orbital diagram:
3d (paired) 4s 4p ↑↓ ↑↓ ↑↓ free free ← d²sp³ hybridisation (inner orbital) → Geometry: Octahedral | Magnetic: Paramagnetic (1 unpaired e⁻, μ = √3 BM)
2 marks
Summary:
Hybridisation: d²sp³ (inner orbital complex)
Geometry: Octahedral
Unpaired electrons: 1 (Fe³⁺ with strong field CN⁻ → 1 unpaired)
Magnetic: Paramagnetic (μ = √3 BM)
Config (1) + Orbital diagram (2) + Hybridisation + Magnetic (2)Total: 5/5
d & f Block
Explain lanthanoid contraction — cause and three consequences
How it's asked
  1. "What is lanthanoid contraction? State its cause and any three consequences." [5]
  2. "Why do the second and third row d-block elements have similar atomic radii? Explain." [5]
Show Model Answer 5 marks
1 mark
Definition: The regular decrease in atomic and ionic radii of lanthanoid elements (La to Lu) with increasing atomic number is called lanthanoid contraction.
2 marks
Cause: 4f electrons have a diffuse orbital with very poor shielding ability. As protons are added (La→Lu), 4f subshell fills but the poor shielding means Z_eff increases significantly with each element → electrons are pulled inward → radius decreases steadily.
2 marks
Three Consequences:
1. Zr and Hf have almost identical radii (Zr: 160 pm, Hf: 159 pm) — very difficult to separate
2. 2nd and 3rd row transition metals (e.g., Mo/W, Nb/Ta) have similar properties — due to lanthanoid contraction compensating for expected increase
3. Basicity of lanthanoid hydroxides decreases La(OH)₃ → Lu(OH)₃ as ionic character decreases
Definition (1) + Cause (2) + 3 Consequences (2)Total: 5/5
Coordination
Crystal Field Theory — explain d-orbital splitting in octahedral complexes
How it's asked
  1. "Using CFT, explain the splitting of d-orbitals in an octahedral crystal field. Distinguish between high-spin and low-spin complexes." [5]
  2. "Why are d-block transition metal complexes coloured? Explain using CFT." [5]
Show Model Answer 5 marks
1 mark
CFT Basis: Ligands are treated as point charges. Their approach causes repulsion with d-electrons. In octahedral field (6 ligands along ±x, ±y, ±z axes), d-orbitals split into 2 groups based on orientation.
2 marks
d-orbital splitting:
Energy Free ion eg (dz², dx²-y²) t₂g (dxy, dxz, dyz) Δo
t₂g: 3 orbitals (0.4Δo below avg); eg: 2 orbitals (0.6Δo above avg)
2 marks
High-spin vs Low-spin & Colour:
Strong field ligands (CN⁻, CO): large Δo → electrons pair in t₂g → low spin
Weak field ligands (F⁻, H₂O): small Δo → electrons go to eg → high spin

Colour: Δo corresponds to visible light energy. Electron absorbs light to jump from t₂g to eg → complementary colour transmitted/observed. E.g., [Ti(H₂O)₆]³⁺ absorbs green, appears violet.
CFT basis (1) + Splitting diagram (2) + High/low spin + colour (2)Total: 5/5