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Class 12 Chemistry Notes Chapter 8 (THe d- and f- Block Elements) – Examplar Problems Book

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Philoid

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Examplar Problems
Detailed Notes with MCQs of Chapter 8: The d- and f- Block Elements. This is a crucial chapter, not just for board exams but also for various government competitive exams where chemistry is a component. We'll break down the key concepts systematically.

Chapter 8: The d- and f- Block Elements - Detailed Notes

1. Introduction

  • d-Block Elements: Elements in which the last electron enters the d-orbital of the penultimate energy shell (n-1)d. They are placed in groups 3 to 12 of the periodic table.
  • Transition Elements: Defined as elements having incompletely filled d-orbitals either in their ground state or in any one of their common oxidation states.
    • Important Note: Zinc (Zn), Cadmium (Cd), and Mercury (Hg) belong to the d-block but are not considered typical transition elements because they have completely filled d-orbitals (d¹⁰ configuration) in their ground state as well as in their common oxidation state (+2).
  • f-Block Elements: Elements in which the last electron enters the f-orbital of the anti-penultimate energy shell (n-2)f. They are placed separately at the bottom of the periodic table.
    • Lanthanoids: 4f-series (following Lanthanum, La). Cerium (Ce, Z=58) to Lutetium (Lu, Z=71).
    • Actinoids: 5f-series (following Actinium, Ac). Thorium (Th, Z=90) to Lawrencium (Lr, Z=103).
    • They are also called Inner Transition Elements.

2. The d-Block Elements (Transition Metals)

(a) Electronic Configuration:

  • General configuration: (n-1)d¹⁻¹⁰ ns¹⁻²
  • Slight variations occur due to the small energy difference between (n-1)d and ns orbitals, leading to extra stability associated with half-filled (d⁵) and completely filled (d¹⁰) configurations.
  • Exceptions: Cr (Z=24): [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²); Cu (Z=29): [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²). Similar patterns exist in subsequent periods.

(b) General Properties & Trends:

  • Metallic Character: All are metals. They are hard, lustrous, malleable, ductile, possess high tensile strength, and are good conductors of heat and electricity. Exceptions: Zn, Cd, Hg are softer. Mercury is liquid at room temperature.
  • Atomic and Ionic Radii:
    • Across a period (e.g., 3d series): Radii decrease initially due to increasing nuclear charge. In the middle, the increase in shielding effect by (n-1)d electrons roughly balances the increased nuclear charge, so radii become almost constant. Towards the end, electron-electron repulsion in d-orbitals may cause a slight increase (e.g., Zn).
    • Down a group: Radii increase from 3d to 4d series as expected. However, radii of 4d and 5d series elements are very similar. This is due to Lanthanoid Contraction.
  • Lanthanoid Contraction: The steady decrease in atomic and ionic radii of lanthanoid elements with increasing atomic number.
    • Cause: Imperfect shielding of the nuclear charge by 4f electrons. As electrons are added to the 4f subshell across the lanthanoid series, the nuclear charge increases, but the shielding effect of 4f electrons is poor. This results in a stronger attraction of the outer electrons by the nucleus, causing a contraction in size.
    • Consequences:
      1. Similarity in properties of second (4d) and third (5d) transition series elements (e.g., Zr and Hf have almost identical radii and properties).
      2. Difficulty in the separation of lanthanoids.
      3. Basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃.
  • Ionization Enthalpy (IE):
    • Generally increases across a period, but irregularities exist due to variations in electronic configurations and stability of resulting ions (e.g., high IE for configurations leading to d⁰, d⁵, d¹⁰).
    • IE values are intermediate between s-block and p-block elements.
    • IE values down a group show less regular trends compared to s- and p-blocks, especially between 4d and 5d series (due to Lanthanoid Contraction, 5d elements have higher IE than expected).
  • Oxidation States:
    • Show variable oxidation states. This is because the energy difference between (n-1)d and ns orbitals is small, allowing both sets of electrons to participate in bonding.
    • The minimum oxidation state is usually +1 or +2 (corresponding to the loss of ns electrons).
    • The maximum oxidation state often corresponds to the sum of ns and (n-1)d electrons (e.g., Mn shows up to +7). Highest oxidation states are typically observed in compounds with highly electronegative elements like Oxygen (O) and Fluorine (F).
    • Stability of oxidation states depends on factors like electronic configuration, lattice energy, hydration enthalpy, and ligand environment. E.g., Mn²⁺ (d⁵) is more stable than Mn³⁺. Cr³⁺ (t₂g³) is stable in octahedral fields. Cu²⁺ is more stable than Cu⁺ in aqueous solution due to high hydration enthalpy, despite Cu⁺ having a d¹⁰ configuration.
    • Lower oxidation states (+2, +3) usually form ionic compounds, while higher oxidation states form covalent compounds (e.g., MnO vs Mn₂O₇).
  • Magnetic Properties:
    • Most transition metal ions and compounds are paramagnetic.
    • Paramagnetism: Attracted by a magnetic field; arises due to the presence of unpaired electrons.
    • Diamagnetism: Repelled by a magnetic field; arises when all electrons are paired.
    • Ferromagnetism: Strong form of paramagnetism (e.g., Fe, Co, Ni).
    • Magnetic moment (µ) is calculated using the 'spin-only' formula:
      µ = √[n(n+2)] Bohr Magnetons (BM), where 'n' is the number of unpaired electrons.
  • Formation of Coloured Ions:
    • Most transition metal compounds are coloured in the solid state or in solution.
    • Cause: Presence of incompletely filled d-orbitals allows for d-d transitions. When light falls on the compound, electrons absorb energy corresponding to a specific colour (wavelength) and get excited from a lower energy d-orbital to a higher energy d-orbital. The transmitted or reflected light is the complementary colour, which is the colour observed.
    • Conditions: Partially filled d-orbitals (d¹ to d⁹).
    • Exceptions: Sc³⁺ (d⁰), Ti⁴⁺ (d⁰), Zn²⁺ (d¹⁰), Cu⁺ (d¹⁰) ions are colourless.
    • Colour also depends on the ligands, geometry of the complex, and oxidation state.
    • Charge transfer spectra can also cause intense colour (e.g., MnO₄⁻, CrO₄²⁻), even with d⁰ configuration in the metal ion.
  • Catalytic Properties:
    • Transition metals and their compounds act as excellent catalysts (e.g., Fe in Haber's process, V₂O₅ in Contact process, Ni in hydrogenation).
    • Reasons:
      1. Ability to exhibit variable oxidation states, allowing them to form intermediate compounds.
      2. Ability to provide a suitable surface area for adsorption of reactants.
      3. Ability to form coordination complexes.
  • Formation of Complex Compounds:
    • Transition metals form a large number of coordination complexes.
    • Reasons:
      1. Relatively small size of metal ions.
      2. High ionic charge.
      3. Availability of vacant d-orbitals to accept lone pairs of electrons from ligands.
  • Formation of Interstitial Compounds:
    • Formed when small non-metal atoms (like H, C, N, B) get trapped in the interstitial voids (empty spaces) within the crystal lattice of transition metals.
    • Typically non-stoichiometric.
    • Properties: Very hard and rigid, high melting points (higher than parent metal), retain metallic conductivity, chemically inert. (e.g., TiC, Fe₃H, VH₀.₅₆).
  • Alloy Formation:
    • Transition metals readily form alloys (homogeneous solid solutions of two or more metals).
    • Reason: Similar atomic sizes allow atoms of one metal to substitute atoms of another metal in the crystal lattice.
    • Examples: Brass (Cu, Zn), Bronze (Cu, Sn), Stainless Steel (Fe, Cr, Ni, C).

(c) Important Compounds:

  • Potassium Dichromate (K₂Cr₂O₇):
    • Preparation: From chromite ore (FeCr₂O₄). Involves fusion with Na₂CO₃/NaOH in air (forming sodium chromate), acidification (forming sodium dichromate), and treatment with KCl.
    • Structure: Dichromate ion (Cr₂O₇²⁻) consists of two tetrahedra sharing one oxygen atom corner. Cr is in +6 oxidation state.
    • Properties: Orange crystalline solid. Strong oxidizing agent, especially in acidic medium.
      Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V)
    • Uses: Oxidizing agent in volumetric analysis (titration of Fe²⁺, I⁻), preparation of organic compounds, leather tanning, chrome plating.
  • Potassium Permanganate (KMnO₄):
    • Preparation: From pyrolusite ore (MnO₂). Involves fusion with KOH in presence of air/oxidizing agent (forming potassium manganate K₂MnO₄, green), followed by electrolytic oxidation or oxidation by Cl₂/O₃/CO₂ in neutral/acidic solution.
    • Structure: Permanganate ion (MnO₄⁻) is tetrahedral. Mn is in +7 oxidation state.
    • Properties: Dark purple (almost black) crystalline solid. Very strong oxidizing agent in acidic, neutral, and alkaline media.
      • Acidic: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.51 V) (Strongest oxidizing action)
      • Neutral/Faintly Alkaline: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻
      • Strongly Alkaline: MnO₄⁻ + e⁻ → MnO₄²⁻ (manganate)
    • Uses: Oxidizing agent in lab and industry, volumetric analysis (titration of Fe²⁺, C₂O₄²⁻, SO₂), disinfectant, water purification (Baeyer's reagent for detecting unsaturation).

3. The f-Block Elements (Inner Transition Metals)

(a) Lanthanoids (4f series):

  • Electronic Configuration: General configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Irregularities exist (e.g., Gd has 4f⁷ 5d¹ 6s²).
  • Oxidation State: Predominantly +3 is the most stable and common oxidation state for all lanthanoids. Some elements show +2 (e.g., Eu²⁺, Yb²⁺ - related to achieving f⁷ or f¹⁴ configuration) and +4 (e.g., Ce⁴⁺ - related to achieving f⁰ configuration).
  • Lanthanoid Contraction: (Discussed earlier under d-block consequences). The effect is significant within the series itself.
  • General Characteristics:
    • Silvery-white, soft metals. Hardness increases with atomic number.
    • High melting and boiling points.
    • Good conductors of heat and electricity.
    • Chemically reactive; reactivity decreases slightly with increasing atomic number. React with H₂O, acids, halogens, non-metals.
    • Form coloured ions (except La³⁺, Lu³⁺, Ce⁴⁺, Yb²⁺). Colour arises from f-f transitions (though mechanism is slightly different and colours are often less intense than d-d transitions).
    • Paramagnetic (except La³⁺, Lu³⁺, Ce⁴⁺, Yb²⁺). Magnetic moments do not follow the simple spin-only formula due to significant orbital contribution.
  • Uses: Production of alloy steels (e.g., Mischmetal - ~95% lanthanoid metals + ~5% Fe + traces of S, C, Ca, Al - used in Mg-based alloys for bullets, shells, lighter flints), catalysts (petroleum cracking), phosphors in television screens and fluorescent lamps, lasers (Nd-YAG laser).

(b) Actinoids (5f series):

  • Electronic Configuration: General configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². More irregularities than lanthanoids due to comparable energies of 5f, 6d, and 7s subshells.
  • Oxidation States: Show a greater range of oxidation states than lanthanoids. +3 is common, but higher states like +4, +5, +6, +7 (for Np, Pu) are also observed. This is because 5f, 6d, and 7s orbitals have comparable energies, allowing more electrons to participate in bonding. The range decreases for heavier actinoids.
  • Actinoid Contraction: Similar to lanthanoid contraction but the decrease in size is greater from element to element due to poorer shielding by 5f electrons compared to 4f electrons.
  • General Characteristics:
    • All are radioactive; earlier members have long half-lives, later ones have very short half-lives.
    • Silvery appearance, but tarnish in air.
    • Highly reactive metals, especially when finely divided.
    • Most ions are coloured (due to 5f-5f or 5f-6d transitions).
    • Paramagnetic. Magnetic properties are more complex than lanthanoids.
    • Exhibit more complex chemical behaviour than lanthanoids due to the wider range of oxidation states.
  • Comparison with Lanthanoids: Actinoids show greater range of oxidation states, are radioactive, have more complex magnetic and spectral properties, and generally form less stable complexes compared to lanthanoids in the +3 state. Actinoid contraction is stronger.
  • Uses: Primarily used in nuclear reactors (U, Pu) and nuclear weapons. Thorium is used in atomic reactors and incandescent gas mantles. Some isotopes are used in cancer therapy and as power sources.

4. Applications of d- and f-Block Elements

  • Structural Materials: Fe, Ti, Cr, Ni, Mn used in construction, machinery, vehicles (often as alloys like steel).
  • Catalysis: V₂O₅, Fe, Ni, Pt, Pd, K₂Cr₂O₇, KMnO₄ used extensively.
  • Electrical Industry: Cu, Ag (best conductor), Hg (thermometers, barometers).
  • Pigments & Colours: TiO₂, ZnO, compounds of Cr, Mn, Fe, Cu.
  • Batteries: Zn, Ni, Cd, MnO₂ (dry cells).
  • Coinage Metals: Cu, Ag, Au (though less common now).
  • Nuclear Energy: U, Pu, Th.
  • Specialty Applications: Lanthanoids in magnets, lasers, phosphors.

Multiple Choice Questions (MCQs)

  1. Which of the following elements is NOT considered a typical transition element?
    (a) Iron (Fe)
    (b) Copper (Cu)
    (c) Zinc (Zn)
    (d) Chromium (Cr)

  2. The electronic configuration of Cr³⁺ ion is:
    (a) [Ar] 3d⁵ 4s¹
    (b) [Ar] 3d⁴
    (c) [Ar] 3d³
    (d) [Ar] 3d⁵

  3. Lanthanoid contraction is primarily caused by:
    (a) The effective shielding of nuclear charge by 4f electrons.
    (b) The poor shielding of nuclear charge by 4f electrons.
    (c) The participation of 5d electrons in bonding.
    (d) The high ionization enthalpy of lanthanoids.

  4. Which of the following pairs of elements have nearly identical atomic radii?
    (a) Fe and Ru
    (b) Cu and Ag
    (c) Zr and Hf
    (d) Cr and Mo

  5. The maximum oxidation state shown by Manganese (Mn, Z=25) is:
    (a) +2
    (b) +4
    (c) +5
    (d) +7

  6. Which of the following ions is expected to be colourless in aqueous solution?
    (a) Fe³⁺
    (b) Mn²⁺
    (c) Sc³⁺
    (d) Ni²⁺

  7. Interstitial compounds are formed when small atoms like H, C, or N are trapped in the crystal lattice of transition metals. These compounds are typically:
    (a) Soft and have low melting points.
    (b) Non-conductors of electricity.
    (c) Chemically very reactive.
    (d) Very hard and have high melting points.

  8. Potassium dichromate (K₂Cr₂O₇) acts as a strong oxidizing agent in acidic medium. The oxidation state of Chromium changes from:
    (a) +6 to +3
    (b) +7 to +2
    (c) +6 to +4
    (d) +4 to +2

  9. Which statement is generally TRUE for Actinoids compared to Lanthanoids?
    (a) Actinoids exhibit a smaller range of oxidation states.
    (b) Actinoids are all non-radioactive.
    (c) Actinoid contraction is less pronounced than lanthanoid contraction.
    (d) Actinoids show more complex chemical behaviour due to lower energy difference between 5f, 6d and 7s orbitals.

  10. The magnetic moment (spin-only) of Ti³⁺ ion (Z=22) is:
    (a) √3 BM
    (b) √8 BM
    (c) √15 BM
    (d) 0 BM

Answer Key for MCQs:

  1. (c)
  2. (c) [Cr: [Ar] 3d⁵ 4s¹ → Cr³⁺: [Ar] 3d³]
  3. (b)
  4. (c) [Due to Lanthanoid Contraction]
  5. (d) [Mn: [Ar] 3d⁵ 4s² → Max OS = 5+2 = +7]
  6. (c) [Sc³⁺ has d⁰ configuration]
  7. (d)
  8. (a) [Cr in Cr₂O₇²⁻ is +6; in acidic medium it reduces to Cr³⁺]
  9. (d)
  10. (a) [Ti: [Ar] 3d² 4s² → Ti³⁺: [Ar] 3d¹ → n=1. µ = √[1(1+2)] = √3 BM]

Study these notes thoroughly. Pay special attention to the trends, reasons behind them (especially electronic configuration and lanthanoid contraction), properties of KMnO₄ and K₂Cr₂O₇, and the comparison between Lanthanoids and Actinoids. These are frequently tested areas. Good luck with your preparation!

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