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Class 12 Chemistry Notes

Inorganic Chemistry Class 12 Notes

Complete, exam-ready notes on inorganic chemistry for Class 12: p-block and d-block elements, f-block (lanthanides and actinides), coordination compounds and crystal field theory — written for CBSE boards, JEE and NEET revision.

Class12SubjectChemistryCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What is inorganic chemistry in one line?

Inorganic chemistry is the study of the properties and reactions of the elements (except carbon compounds) — including p-block, d-block and f-block elements, and coordination compounds.

p-Block Elements

  • Group 15 (N, P, As, Sb, Bi): nitrogen is a gas, P and As solids; hydrides NH₃, PH₃, etc. Ammonia is the strongest base.
  • Group 16 (O, S, Se, Te): ozone O₃ is an allotrope; sulphur forms many allotropes (rhombic, monoclinic); H₂SO₄ is a strong dibasic acid.
  • Group 17 (halogens): reactivity decreases down the group; fluorine is the most electronegative element.
  • Group 18 (noble gases): chemically inert, with Xe forming stable compounds with F and O (e.g. XeF₂, XeF₄).

Key anomalies

HF forms hydrogen-bonded polymers and is a weak acid despite being a halogen acid. Nitrogen lacks vacant d-orbitals so it does not form pentahalides or show the acidic/basic dual behaviour of phosphorus.

d-Block (Transition) Elements

Transition element

An element with a partially filled d-orbital in its ion or ground state. They show variable oxidation states, form coloured ions and complexes, and are useful catalysts.

  • Variable oxidation states — e.g. Mn shows +2 to +7.
  • Colour arises from d-d transitions.
  • Magnetic properties from unpaired d-electrons.
  • Catalytic activity: Fe in Haber process, V₂O₅ in contact process, Ni in hydrogenation.
  • High melting points due to strong metallic bonding from d-electrons.

Sc and Zn are not typical transition elements

Sc³⁺ has an empty d-orbital (3d⁰) and Zn²⁺ has a full d-orbital (3d¹⁰), so neither shows the usual transition-metal properties like variable oxidation states or coloured ions.

f-Block Elements (Lanthanides and Actinides)

  • Lanthanides (Ce–Lu): show +3 oxidation state predominantly, lanthanide contraction (steady decrease in ionic radius with atomic number).
  • Actinides (Th–Lr): radioactive, show variable oxidation states.
  • Lanthanide contraction arises because the 4f electrons poorly shield the increasing nuclear charge.
  • Uses: mischmetal in lighter flints, lanthanides in magnets and phosphors.

Coordination Compounds

Coordination compound

[M(L)n]x[M(L)_n]^x

A complex formed by a central metal ion (M) bonded to ligands (L). The coordination number is the number of donor atoms bonded to the metal. Common geometries: tetrahedral (4), square planar (4), octahedral (6).

Crystal field theory (CFT)

Ligands create a crystal field that splits the degenerate d-orbitals into t₂g (lower energy) and e_g (higher energy) sets in octahedral complexes. The splitting energy is Δo\Delta_o.

  • Strong-field ligands (CN⁻, CO) cause large Δo and give low-spin complexes.
  • Weak-field ligands (Cl⁻, H₂O) cause small Δo and give high-spin complexes.
  • Spectrochemical series:
  • I−<Br−<Cl−<F−<OH−<H2O<NH3<en<CN−<CO\text{I}^- < \text{Br}^- < \text{Cl}^- < \text{F}^- < \text{OH}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}
  • .
  • Werner's theory explained primary (ionisable) vs secondary (coordination) valence.

Isomerism in Coordination Compounds

  • Geometrical (cis/trans) — depends on arrangement of ligands.
  • Optical (d/l) — non-superimposable mirror images.
  • Linkage isomerism — ligand binds through different donor atoms (e.g. -NO₂ vs -ONO).
  • Ionisation isomerism — different ions in the coordination sphere vs solution.
  • Coordination isomerism — exchange of ligands between cationic and anionic complexes.

Qualitative and Quantitative Analysis

Inorganic qualitative analysis uses group reagents to precipitate cations: H₂S, NH₄OH and (NH₄)₂S separate ions into groups. Volumetric analysis includes redox titrations (KMnO₄, K₂Cr₂O₇) and gravimetric methods.

  • Cation group separation based on solubility products of sulphides and hydroxides.
  • Confirmatory tests: flame test, borax bead test, sodium carbonate extract tests.
  • Redox titrations: self-indicating KMnO₄ (pink) and K₂Cr₂O₇ with diphenylamine indicator.

Solved Examples

Example: Find the oxidation state of Cr in [Cr(NH3)4Cl2]+[\text{Cr(NH}_3)_4\text{Cl}_2]^+.

Solution: NH₃ is neutral, Cl is -1 (two Cl = -2), overall charge +1, so x+0−2=+1⇒x=+3x + 0 - 2 = +1 \Rightarrow x = +3. Cr is in the +3 oxidation state.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Spectrochemical series

I−<⋯<H2O<NH3<en<CN−<CO\text{I}^- < \dots < \text{H}_2\text{O} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}

CFT splitting (octahedral)

d-orbitals split by Δod \text{-orbitals split by } \Delta_o

Coordination compound

[M(L)n]x[M(L)_n]^x

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

  • Sc and Zn are not typical transition elements.
  • Strong-field ligands form low-spin complexes.
  • Lanthanide contraction explains similar properties across the series.
  • Spectrochemical series predicts field strength and spin state.
  • NH₃ is neutral, so oxidation state of Cr in [Cr(NH₃)₄Cl₂]+ is +3.

FAQ

Common questions

What is the spectrochemical series?

It is a ranking of ligands by the magnitude of crystal-field splitting they cause: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO. CN⁻ and CO are strong-field ligands.

Why do transition metals form coloured compounds?

Because of d-d electronic transitions between split d-orbitals, which absorb visible light. This is why ions like Cu²⁺ and Cr³⁺ appear coloured.

What is the difference between high-spin and low-spin complexes?

Weak-field ligands (small Δo) give high-spin complexes where electrons fill d-orbitals singly; strong-field ligands (large Δo) give low-spin complexes where electrons pair in lower-energy t₂g orbitals.

What is lanthanide contraction?

The steady decrease in ionic radius across the lanthanide series due to the poor shielding of 4f electrons. It causes chemical similarity across consecutive lanthanides.

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All Class 12 NotesClass 12 Solutions NotesClass 12 Solid State NotesChemistry Tutor in Mohali

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