The d- and f-Block Elements
Welcome to HSLC Guru. This page provides a complete English-medium study guide for ASSEB Class 12 Chemistry Chapter 8 — The d- and f-Block Elements. The chapter introduces the transition metals (d-block) and inner-transition metals (f-block), exploring their electronic configurations, characteristic properties, important compounds, and applications. Use the summary, solved questions, MCQs, and glossary below to prepare effectively for your HSLC examination.
Chapter Summary
The d-block elements, also called transition elements, occupy the central block of the periodic table between the s-block and p-block. They are defined as elements whose atoms (or stable ions) have partially filled d-orbitals. There are four series of d-block elements: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg), and 6d (incomplete). Their general electronic configuration is (n-1)d¹⁻¹⁰ ns⁰⁻². Although Zn, Cd, and Hg have completely filled d-orbitals (d¹⁰), they are usually studied along with transition elements because of similarities in their properties.
Transition metals exhibit several characteristic properties: variable oxidation states due to similar energies of ns and (n-1)d electrons; coloured ions arising from d-d electronic transitions in partially filled d-orbitals; paramagnetism due to unpaired electrons (magnetic moment μ = √n(n+2) BM, where n is the number of unpaired electrons); strong tendency to form complex compounds due to small size, high charge, and availability of vacant d-orbitals; excellent catalytic activity (e.g., Fe in Haber’s process, V₂O₅ in contact process, Ni in hydrogenation); and formation of alloys and interstitial compounds with non-metals like H, B, C, N. They generally have high melting and boiling points, high density, and high enthalpies of atomisation.
Two important compounds are potassium dichromate (K₂Cr₂O₇), prepared from chromite ore (FeCr₂O₄) — it is a powerful oxidising agent in acidic medium and is used in leather tanning, dyeing, and as a primary standard in volumetric analysis; and potassium permanganate (KMnO₄), prepared from pyrolusite (MnO₂) — it is a strong oxidising agent in acidic, neutral, and alkaline media, used as a disinfectant, in titrations, and in organic synthesis. Their properties depend strongly on the pH of the medium.
The f-block elements or inner-transition elements include the lanthanides (4f series, Ce to Lu) and actinides (5f series, Th to Lr). Their general configuration is (n-2)f¹⁻¹⁴ (n-1)d⁰⁻¹ ns². The lanthanide contraction — the gradual decrease in atomic and ionic radii across the lanthanide series due to poor shielding by 4f electrons — has important consequences: similar sizes of 4d and 5d elements, similarity in properties of lanthanides making their separation difficult, and increased basicity from La³⁺ to Lu³⁺ decreasing. Actinides differ from lanthanides in showing a wider range of oxidation states, greater tendency to form complexes, and being radioactive. Lanthanides are used in alloys (mischmetal), catalysts, and phosphors; actinides like uranium and plutonium are used as nuclear fuels.
1-Mark Questions and Answers
Q1. What are d-block elements?
Answer: Elements in which the last electron enters the (n-1)d orbital are called d-block elements. They lie between the s-block and p-block in the periodic table.
Q2. Write the general electronic configuration of d-block elements.
Answer: The general electronic configuration is (n-1)d¹⁻¹⁰ ns⁰⁻², where n is the outermost shell.
Q3. Why are Zn, Cd, and Hg not considered typical transition elements?
Answer: Because they have completely filled d-orbitals (d¹⁰) in both their atomic and common ionic states, so they do not show typical transition metal properties.
Q4. Why do transition metal ions show colour?
Answer: Due to d-d electronic transitions, where electrons absorb visible light to jump between split d-orbitals in a ligand field.
Q5. What is lanthanide contraction?
Answer: The gradual decrease in atomic and ionic radii of lanthanides with increasing atomic number is called lanthanide contraction, caused by poor shielding of 4f electrons.
Q6. Name the most common oxidation state of lanthanides.
Answer: The most common and stable oxidation state of lanthanides is +3.
Q7. Write the formula of potassium dichromate.
Answer: The formula is K₂Cr₂O₇.
Q8. What is mischmetal?
Answer: Mischmetal is an alloy containing about 95% lanthanide metals (mainly Ce) and 5% iron, with traces of S, C, Ca, Al; used in flints of cigarette lighters and tracer bullets.
Q9. Which transition metal is used as catalyst in Haber’s process?
Answer: Iron (Fe) is used as a catalyst with molybdenum as a promoter in Haber’s process.
Q10. What is the oxidation state of Mn in KMnO₄?
Answer: The oxidation state of Mn in KMnO₄ is +7.
2-3 Mark Questions and Answers
Q1. Why do transition metals show variable oxidation states?
Answer: Transition metals show variable oxidation states because the energies of ns and (n-1)d orbitals are very close to each other. Therefore, electrons from both ns and (n-1)d orbitals can take part in bond formation. The number of unpaired d-electrons available decides the maximum oxidation state. For example, Mn ([Ar]3d⁵4s²) shows oxidation states from +2 to +7. The lower oxidation states are usually ionic while higher ones are covalent and often present in oxoanions.
Q2. Why are transition metals good catalysts?
Answer: Transition metals act as good catalysts because (i) they show variable oxidation states which allow them to form intermediate compounds easily, (ii) they have vacant d-orbitals that can accept electrons from reactants, (iii) they provide a large surface area for adsorption of reactant molecules, and (iv) they can change their oxidation state during the reaction. Examples: V₂O₅ in contact process, Fe in Haber’s process, Ni in hydrogenation of oils.
Q3. What are interstitial compounds? Give examples.
Answer: Interstitial compounds are formed when small atoms like H, B, C, N are trapped inside the crystal lattice (interstitial spaces) of transition metals. They are non-stoichiometric and neither typically ionic nor covalent. They are very hard, have high melting points, retain metallic conductivity, and are chemically inert. Examples: TiC, Mn₄N, Fe₃H, VH₀.₅₆, TiH₁.₇.
Q4. Calculate the magnetic moment of Fe³⁺ ion.
Answer: Configuration of Fe³⁺: [Ar] 3d⁵. Number of unpaired electrons (n) = 5. Magnetic moment μ = √n(n+2) BM = √5(5+2) = √35 = 5.92 BM. So Fe³⁺ is strongly paramagnetic.
Q5. Mention three consequences of lanthanide contraction.
Answer: Three consequences of lanthanide contraction are: (i) The atomic and ionic radii of 4d and 5d transition elements become almost equal (e.g., Zr and Hf), making their separation very difficult. (ii) The basic strength of hydroxides decreases from La(OH)₃ to Lu(OH)₃ as the ionic size decreases. (iii) The properties of lanthanides are very similar, making their chemical separation difficult and requiring techniques like ion exchange.
Q6. Why is Cu²⁺ coloured but Zn²⁺ is colourless?
Answer: Cu²⁺ has the configuration [Ar]3d⁹ with one unpaired electron in the d-orbital. It can absorb visible light to undergo d-d transition, hence appears blue. Zn²⁺ has the configuration [Ar]3d¹⁰ with completely filled d-orbitals; no d-d transition is possible, so it is colourless.
5-7 Mark Questions and Answers
Q1. Discuss the general characteristics of transition elements.
Answer: The general characteristics of transition elements are:
(i) Metallic Character: All are typical metals — hard, malleable, ductile, with high tensile strength and good electrical/thermal conductivity. Mercury is the only liquid metal.
(ii) High Melting and Boiling Points: Due to strong metallic bonds involving both ns and (n-1)d electrons. Tungsten has the highest melting point.
(iii) Variable Oxidation States: Due to close energies of ns and (n-1)d electrons, multiple oxidation states are exhibited. Mn shows from +2 to +7.
(iv) Coloured Ions: Most transition metal ions are coloured due to d-d transitions. Sc³⁺ (d⁰) and Zn²⁺ (d¹⁰) are colourless.
(v) Magnetic Properties: Most are paramagnetic due to unpaired d-electrons; some (Fe, Co, Ni) are ferromagnetic.
(vi) Catalytic Activity: Many transition metals/compounds act as catalysts (Fe, Ni, Pt, V₂O₅).
(vii) Complex Formation: Form complexes due to small size, high nuclear charge, and vacant d-orbitals.
(viii) Alloy Formation: Form alloys due to similar atomic sizes (e.g., brass, bronze, steel).
(ix) Interstitial Compounds: Trap small atoms (H, C, N, B) in lattice voids forming hard, high melting compounds.
Q2. Describe the preparation and properties of potassium dichromate (K₂Cr₂O₇).
Answer: Preparation: K₂Cr₂O₇ is prepared from chromite ore (FeCr₂O₄) in three steps:
(i) Fusion with sodium carbonate in air: 4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂.
(ii) Acidification of sodium chromate with H₂SO₄ to form sodium dichromate: 2 Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O.
(iii) Treatment with KCl to obtain K₂Cr₂O₇: Na₂Cr₂O₇ + 2 KCl → K₂Cr₂O₇ + 2 NaCl.
Properties: (i) Orange-red crystalline solid, soluble in water. (ii) On heating, decomposes: 4 K₂Cr₂O₇ → 4 K₂CrO₄ + 2 Cr₂O₃ + 3 O₂. (iii) In acidic medium, it is a strong oxidising agent: Cr₂O₇²⁻ + 14 H⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O. (iv) Equilibrium with chromate: Cr₂O₇²⁻ + H₂O ⇌ 2 CrO₄²⁻ + 2 H⁺ (orange in acidic, yellow in alkaline). Uses: In leather tanning, dyeing, photography, and as a primary standard in titrations.
Q3. Describe the preparation and properties of potassium permanganate (KMnO₄).
Answer: Preparation: KMnO₄ is prepared from pyrolusite (MnO₂) in two steps:
(i) Fusion of MnO₂ with KOH in presence of air or KNO₃: 2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂O. Green potassium manganate is formed.
(ii) Oxidation of K₂MnO₄ either electrolytically or with chlorine/ozone: 2 K₂MnO₄ + Cl₂ → 2 KMnO₄ + 2 KCl.
Properties: (i) Dark purple crystalline solid, moderately soluble in water giving violet solution. (ii) On heating above 513 K: 2 KMnO₄ → K₂MnO₄ + MnO₂ + O₂. (iii) Strong oxidising agent in all media. In acidic medium: MnO₄⁻ + 8 H⁺ + 5 e⁻ → Mn²⁺ + 4 H₂O. In neutral/slightly alkaline: MnO₄⁻ + 2 H₂O + 3 e⁻ → MnO₂ + 4 OH⁻. In strongly alkaline: MnO₄⁻ + e⁻ → MnO₄²⁻. Uses: As disinfectant, in volumetric titrations, in organic synthesis, and in water treatment.
Q4. Compare lanthanides and actinides.
Answer: Comparison between lanthanides and actinides:
Similarities: Both belong to f-block; both show +3 as principal oxidation state; both undergo contraction (lanthanide and actinide contraction); both are typical metals with similar physical properties.
Differences: (i) Lanthanides involve filling of 4f orbitals; actinides involve filling of 5f orbitals. (ii) Lanthanides mostly show +3 oxidation state; actinides show a wider range (+3 to +7). (iii) 5f electrons in actinides are less effectively shielded than 4f, so actinide contraction is greater per element. (iv) Lanthanides are mostly non-radioactive (except Pm); all actinides are radioactive. (v) Actinides have greater tendency to form complexes due to larger charge/size ratio. (vi) Lanthanide compounds are less basic than actinide compounds. (vii) Lanthanide ions are mostly paramagnetic; actinide magnetism is more complex due to 5f-6d interaction.
Q5. Explain the trends in atomic radii, ionisation enthalpy, and oxidation states across the 3d series.
Answer: Atomic Radii: Across the 3d series, atomic radii first decrease (Sc to Cr) due to increase in nuclear charge, then become almost constant from Cr to Cu because the increased nuclear charge is balanced by the screening effect of d-electrons, and finally increase slightly (Zn) due to increased electron-electron repulsion in fully filled d-orbitals.
Ionisation Enthalpy: Ionisation enthalpies generally increase across the period due to increasing nuclear charge but the increase is not regular due to varying stabilities of half-filled and fully-filled d-configurations. Cr (3d⁵4s¹) and Cu (3d¹⁰4s¹) have lower IE₁ values than expected because of these stable configurations.
Oxidation States: Maximum oxidation state increases from Sc (+3) to Mn (+7), then decreases. The +2 state becomes more stable from left to right as removal of 4s electrons becomes easier with increasing nuclear charge. Higher oxidation states are stabilised by oxide and fluoride ions. Mn shows the maximum range (+2 to +7) because all 3d⁵4s² electrons can participate in bonding.
Multiple Choice Questions (MCQs)
Q1. The general electronic configuration of d-block elements is:
(a) ns² np⁶ (b) (n-1)d¹⁻¹⁰ ns⁰⁻² (c) (n-2)f¹⁻¹⁴ (d) ns¹⁻²
Answer: (b) (n-1)d¹⁻¹⁰ ns⁰⁻²
Q2. Which of the following is not a transition metal?
(a) Fe (b) Cu (c) Zn (d) Ca
Answer: (d) Ca
Q3. The colour of transition metal ions is due to:
(a) p-p transition (b) s-p transition (c) d-d transition (d) f-f transition
Answer: (c) d-d transition
Q4. The number of unpaired electrons in Fe³⁺ is:
(a) 3 (b) 4 (c) 5 (d) 6
Answer: (c) 5
Q5. Lanthanide contraction is due to poor shielding by:
(a) s electrons (b) p electrons (c) d electrons (d) f electrons
Answer: (d) f electrons
Q6. The most common oxidation state of lanthanides is:
(a) +1 (b) +2 (c) +3 (d) +4
Answer: (c) +3
Q7. The oxidation state of Cr in K₂Cr₂O₇ is:
(a) +3 (b) +5 (c) +6 (d) +7
Answer: (c) +6
Q8. The catalyst used in Haber’s process is:
(a) V₂O₅ (b) Pt (c) Fe (d) Ni
Answer: (c) Fe
Q9. Which lanthanide is liquid at slightly above room temperature?
(a) Ce (b) Eu (c) Hg (not lanthanide) (d) None
Answer: (d) None (lanthanides are solids; trick question — Hg is a transition metal)
Q10. The colour of KMnO₄ solution is:
(a) Green (b) Violet (c) Orange (d) Yellow
Answer: (b) Violet
Fill in the Blanks
Q1. The general electronic configuration of d-block elements is __________.
Answer: (n-1)d¹⁻¹⁰ ns⁰⁻²
Q2. The colour of transition metal ions is due to __________ transitions.
Answer: d-d
Q3. Lanthanides belong to the __________ series of f-block.
Answer: 4f
Q4. The catalyst used in contact process is __________.
Answer: V₂O₅
Q5. The magnetic moment formula for spin-only paramagnetism is μ = __________ BM.
Answer: √n(n+2)
True or False
Q1. All transition metal ions are coloured.
Answer: False (Sc³⁺ and Zn²⁺ are colourless)
Q2. Lanthanide contraction makes Zr and Hf chemically similar.
Answer: True
Q3. Mercury is a solid metal at room temperature.
Answer: False (it is liquid at room temperature)
Q4. Actinides are all radioactive elements.
Answer: True
Q5. KMnO₄ is a reducing agent in acidic medium.
Answer: False (it is a strong oxidising agent in acidic medium)
Glossary
| Term | Definition |
|---|---|
| d-Block Elements | Elements in which the last electron enters the (n-1)d orbital. |
| Transition Element | Element whose atom or stable ion has partially filled d-orbitals. |
| Inner-Transition Elements | Elements in which the last electron enters the (n-2)f orbital — lanthanides and actinides. |
| Lanthanides | 14 elements from Ce (58) to Lu (71) in the 4f series. |
| Actinides | 14 elements from Th (90) to Lr (103) in the 5f series. |
| Lanthanide Contraction | Gradual decrease in atomic and ionic radii across the lanthanide series due to poor 4f shielding. |
| Variable Oxidation State | Property of showing more than one oxidation state due to similar energies of ns and (n-1)d orbitals. |
| d-d Transition | Excitation of an electron between split d-orbitals causing colour in transition metal complexes. |
| Paramagnetism | Property of being attracted by a magnetic field due to presence of unpaired electrons. |
| Magnetic Moment | Quantitative measure of paramagnetism, μ = √n(n+2) BM. |
| Complex Compound | Compound consisting of a central metal atom/ion bonded to surrounding ligands. |
| Interstitial Compound | Compound formed by trapping small atoms (H, B, C, N) in the voids of a metal lattice. |
| Alloy | Homogeneous mixture of two or more metals (or a metal with a non-metal). |
| Mischmetal | An alloy of lanthanide metals (mainly Ce) with iron, used in lighter flints. |
| Pyrolusite | An ore of manganese, MnO₂, used to prepare KMnO₄. |
| Chromite | An ore of chromium, FeCr₂O₄, used to prepare K₂Cr₂O₇. |
| Oxidising Agent | Substance that gains electrons and gets reduced; oxidises others. |
| Catalyst | Substance that alters the rate of a reaction without being consumed. |
| Disproportionation | Reaction in which an element in an intermediate oxidation state simultaneously gets oxidised and reduced. |
| Ferromagnetism | Strong magnetic property exhibited by Fe, Co, Ni due to alignment of magnetic moments. |
Quick Recap: The d-block consists of four series with general configuration (n-1)d¹⁻¹⁰ ns⁰⁻². Key properties: variable oxidation states, colour from d-d transitions, paramagnetism, complex formation, catalysis, and alloy/interstitial compound formation. The f-block — lanthanides (4f) and actinides (5f) — show inner-transition behaviour, with lanthanide contraction shaping the chemistry of post-lanthanide d-block elements.
Exam Tip: Practise writing electronic configurations, calculating magnetic moments, and balancing redox equations for K₂Cr₂O₇ and KMnO₄ in different media. These are frequently asked in ASSEB HSLC examinations.
End of Chapter 8 — The d- and f-Block Elements. Continue your preparation with HSLC Guru and master ASSEB Class 12 Chemistry.