Class 12 Chemistry Notes Chapter 8 (The d- and f-block elements) – Chemistry-I Book
Detailed Notes with MCQs of Chapter 8: The d- and f-Block Elements. This is a crucial chapter, not just for your board exams but also for various government competitive exams where chemistry is a component. Pay close attention to the trends, exceptions, and specific properties.
Chapter 8: The d- and f-Block Elements - Detailed Notes
1. Introduction
- d-Block Elements (Transition Elements):
- Located between s-block and p-block elements (Groups 3 to 12).
- Defined as elements having incompletely filled d-orbitals in their ground state or in any one of their common oxidation states.
- Characterized by the filling of penultimate d-orbitals ((n-1)d).
- General Electronic Configuration: (n-1)d¹⁻¹⁰ ns¹⁻² (n = 4, 5, 6, 7).
- Four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg), 6d (Ac, Rf onwards - incomplete).
- Note: Zn, Cd, Hg have completely filled d-orbitals (d¹⁰) in their ground state and common +2 oxidation state. Hence, they are often not considered typical transition elements, though studied with them.
- f-Block Elements (Inner Transition Elements):
- Located separately at the bottom of the periodic table.
- Characterized by the filling of anti-penultimate f-orbitals ((n-2)f).
- Two series:
- Lanthanoids (4f series): Ce (Z=58) to Lu (Z=71). Follow Lanthanum (La, Z=57). General Electronic Configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s².
- Actinoids (5f series): Th (Z=90) to Lr (Z=103). Follow Actinium (Ac, Z=89). General Electronic Configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s².
2. Properties of d-Block Elements (Transition Metals)
- Physical Properties:
- Almost all are typical metals: Hard, lustrous, malleable, ductile, high tensile strength, high melting and boiling points, good thermal and electrical conductivity.
- Exceptions: Zn, Cd, Hg are softer and have lower melting points due to the absence of unpaired d-electrons contributing to metallic bonding. Mercury is liquid at room temperature.
- High Melting/Boiling Points: Due to strong metallic bonding involving both ns and (n-1)d electrons. Maxima generally occur around the middle of the series (e.g., Cr, Mo, W) where unpaired d-electrons are maximal.
- Atomic and Ionic Radii:
- Across a Period (e.g., 3d series): Radii decrease from left to right initially due to increasing effective nuclear charge. Towards the middle, the decrease becomes smaller due to the shielding effect of (n-1)d electrons partially counteracting the increased nuclear charge. At the end (e.g., Cu, Zn), size increases slightly due to increased electron-electron repulsion in the d-orbitals.
- Down a Group: Radii increase from 3d to 4d series as expected. However, the radii of the 4d and 5d series elements in the same group are very similar.
- 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 series, the nuclear charge increases, but the shielding effect of 4f electrons is poor, leading to a stronger pull on the outer electrons.
- Consequences:
- Remarkable similarity in the atomic radii (and chemical properties) of the second (4d) and third (5d) transition series elements (e.g., Zr/Hf, Nb/Ta, Mo/W).
- Difficulty in the separation of lanthanoids.
- Basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃.
- Ionization Enthalpies (IE):
- Generally increase across a period, but the increase is less steep than for s- or p-block elements.
- Irregularities exist due to variations in stability associated with electronic configurations (e.g., half-filled d⁵ or fully-filled d¹⁰). For example, Cr and Cu have high second IE due to the stability of Cr⁺ (d⁵) and Cu⁺ (d¹⁰). Zn has very high third IE because it involves removing an electron from the stable Zn²⁺ (d¹⁰) configuration.
- IE values generally decrease down a group, but the 5d elements often have higher IE₁ than 3d and 4d elements due to Lanthanoid Contraction (stronger effective nuclear charge).
- Oxidation States:
- Exhibit variable oxidation states.
- Reason: Small energy difference between ns and (n-1)d subshells, allowing electrons from both to participate in bonding.
- Lowest oxidation state is usually +1 or +2 (loss of ns electrons).
- Highest oxidation state often corresponds to the group number (up to Mn, Group 7). E.g., Sc (+3), Ti (+4), V (+5), Cr (+6), Mn (+7). Beyond Mn, higher states (+8 for Ru, Os) are seen but not common for Fe, Co, Ni.
- Stability of oxidation states depends on factors like electronic configuration, lattice energy, hydration enthalpy, and nature of bonding (ionic/covalent).
- Higher oxidation states are generally stabilized in fluorides and oxides due to the high electronegativity and small size of F and O. Example: VF₅, CrO₃, Mn₂O₇.
- Lower oxidation states (+2, +3) usually form ionic compounds, while higher oxidation states tend to form covalent compounds (e.g., MnO₄⁻, CrO₄²⁻).
- Magnetic Properties:
- Most transition metal ions and compounds are paramagnetic.
- Reason: Presence of unpaired electrons in (n-1)d orbitals. These unpaired electrons have a magnetic moment associated with their spin and orbital motion.
- Diamagnetic: Substances repelled by a magnetic field (e.g., Sc³⁺, Ti⁴⁺, Zn²⁺, Cu⁺ - all d⁰ or d¹⁰).
- Paramagnetic: Substances attracted by a magnetic field.
- Magnetic Moment (µ): Calculated using the 'spin-only' formula:
µ = √[n(n+2)] Bohr Magnetons (BM)
where 'n' is the number of unpaired electrons. - Ferromagnetism: Strong form of paramagnetism shown by Fe, Co, Ni.
- Formation of Coloured Ions:
- Most transition metal compounds are coloured in the solid state or in solution.
- Reason: Presence of incompletely filled d-orbitals allows for d-d transitions. When visible light falls on the compound, electrons absorb energy of a specific wavelength (colour) and get excited from a lower energy d-orbital to a higher energy d-orbital. The transmitted light is the complementary colour, which is the colour observed.
- Exceptions: Sc³⁺ (3d⁰), Ti⁴⁺ (3d⁰), Zn²⁺ (3d¹⁰), Cu⁺ (3d¹⁰) are colourless as d-d transitions are not possible.
- Colour can also arise due to charge transfer spectra (e.g., MnO₄⁻, CrO₄²⁻), which are often very intense.
- Formation of Complex Compounds:
- Transition metals form a large number of coordination compounds (complexes).
- Reasons:
- Relatively small size of metal ions.
- High ionic charges.
- Availability of vacant d-orbitals for accepting lone pairs of electrons from ligands.
- Catalytic Properties:
- Many transition metals and their compounds act as efficient catalysts.
- Reasons:
- Ability to exhibit variable oxidation states, allowing them to form unstable intermediates and provide new reaction paths with lower activation energy.
- Ability to provide a suitable surface area for reactants to adsorb and react.
- Examples: Fe in Haber's process (N₂ + 3H₂ → 2NH₃), V₂O₅ in Contact process (SO₂ → SO₃), Ni in hydrogenation of oils, TiCl₄ (Ziegler-Natta catalyst) in polymerisation of ethene.
- Formation of Interstitial Compounds:
- Transition metals can trap small non-metal atoms (like H, C, N, B) in the interstitial voids (empty spaces) within their crystal lattices.
- These are typically non-stoichiometric.
- Properties: Very hard, high melting points (often higher than parent metal), retain metallic conductivity, chemically inert. Examples: TiC, Fe₃H, VH₀.₅₆.
- Alloy Formation:
- Transition metals readily form alloys (mixtures of 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). Alloys are often harder, have higher melting points, and are more resistant to corrosion than parent metals.
3. Important Compounds of Transition Elements
- Potassium Dichromate (K₂Cr₂O₇):
- Preparation: From chromite ore (FeCr₂O₄).
- Fusion of chromite ore with Na₂CO₃/K₂CO₃ in excess air:
4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂(Sodium chromate - yellow) - Acidification of sodium chromate solution:
2 Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O(Sodium dichromate - orange) - Treatment with KCl:
Na₂Cr₂O₇ + 2 KCl → K₂Cr₂O₇ + 2 NaCl(Potassium dichromate - less soluble, crystallizes out)
- Fusion of chromite ore with Na₂CO₃/K₂CO₃ in excess air:
- Structure: Dichromate ion (Cr₂O₇²⁻) consists of two tetrahedra sharing one oxygen atom corner. Cr-O terminal bonds are shorter than Cr-O bridging bonds. Cr is in +6 oxidation state.
- Properties: Orange crystalline solid. Strong oxidizing agent, especially in acidic medium.
Cr₂O₇²⁻ + 14 H⁺ + 6 e⁻ → 2 Cr³⁺ + 7 H₂O(E° = +1.33 V) - Oxidizing Actions (Acidic Medium):
- Oxidizes Iodide (I⁻) to Iodine (I₂).
- Oxidizes Iron(II) (Fe²⁺) to Iron(III) (Fe³⁺).
- Oxidizes H₂S to Sulphur (S).
- Oxidizes Sulphite (SO₃²⁻) or SO₂ to Sulphate (SO₄²⁻).
- Oxidizes Nitrite (NO₂⁻) to Nitrate (NO₃⁻).
- Chromyl Chloride Test: Used for detecting chloride ions. Heating a chloride salt with K₂Cr₂O₇ and conc. H₂SO₄ produces orange-red vapours of chromyl chloride (CrO₂Cl₂).
- Preparation: From chromite ore (FeCr₂O₄).
- Potassium Permanganate (KMnO₄):
- Preparation: From pyrolusite ore (MnO₂).
- Fusion of MnO₂ with KOH (or K₂CO₃) in presence of air or an oxidizing agent like KNO₃:
2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂O(Potassium manganate - green) - Oxidation of manganate (MnO₄²⁻) to permanganate (MnO₄⁻) by:
- Electrolytic oxidation in alkaline solution.
- Disproportionation in neutral/acidic solution:
3 MnO₄²⁻ + 4 H⁺ → 2 MnO₄⁻ + MnO₂ + 2 H₂O - Oxidation by Cl₂ or O₃:
2 K₂MnO₄ + Cl₂ → 2 KMnO₄ + 2 KCl
- Fusion of MnO₂ with KOH (or K₂CO₃) in presence of air or an oxidizing agent like KNO₃:
- Structure: Permanganate ion (MnO₄⁻) is tetrahedral. Mn is in +7 oxidation state.
- Properties: Dark purple (almost black) crystalline solid. Strong oxidizing agent in acidic, neutral, and alkaline media.
- Acidic Medium (strongest oxidizing action):
MnO₄⁻ + 8 H⁺ + 5 e⁻ → Mn²⁺ (colourless) + 4 H₂O(E° = +1.51 V)- Oxidizes Fe²⁺ to Fe³⁺.
- Oxidizes I⁻ to I₂.
- Oxidizes Oxalate (C₂O₄²⁻) to CO₂.
- Oxidizes H₂S to S.
- Oxidizes SO₂ to SO₄²⁻.
- Oxidizes Nitrite (NO₂⁻) to Nitrate (NO₃⁻).
- Neutral or Faintly Alkaline Medium:
MnO₄⁻ + 2 H₂O + 3 e⁻ → MnO₂ (brown ppt) + 4 OH⁻- Oxidizes I⁻ to Iodate (IO₃⁻).
- Oxidizes Thiosulphate (S₂O₃²⁻) to Sulphate (SO₄²⁻).
- Oxidizes Mn²⁺ to MnO₂.
- Strongly Alkaline Medium:
MnO₄⁻ + e⁻ → MnO₄²⁻ (manganate - green)
- Acidic Medium (strongest oxidizing action):
- Uses: Volumetric analysis (titrations), disinfectant, water purification (Baeyer's reagent - cold, dilute, alkaline KMnO₄ used to test unsaturation).
- Preparation: From pyrolusite ore (MnO₂).
4. The f-Block Elements (Inner Transition Metals)
- Lanthanoids (4f Series):
- Electronic Configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Exceptions exist (e.g., Gd [Xe] 4f⁷ 5d¹ 6s², Lu [Xe] 4f¹⁴ 5d¹ 6s²) due to stability of half-filled/fully-filled f-orbitals.
- Oxidation State: Predominantly +3. Some elements show +2 (e.g., Eu²⁺, Yb²⁺ - stable f⁷, f¹⁴ configurations) and +4 (e.g., Ce⁴⁺ - stable f⁰ configuration).
- Lanthanoid Contraction: (Already explained under d-block properties). Its effects are significant here.
- General Characteristics:
- Silvery-white, soft metals. Hardness increases with atomic number.
- High melting and boiling points.
- Good conductors of heat and electricity.
- Tarnish readily in air. React with water (slowly with cold, rapidly with hot).
- Most trivalent lanthanoid ions (Ln³⁺) are coloured (due to f-f transitions). Exceptions: La³⁺ (4f⁰), Lu³⁺ (4f¹⁴) are colourless.
- Paramagnetic (except La³⁺, Ce⁴⁺, Yb²⁺, Lu³⁺). Magnetic moments do not follow the simple spin-only formula.
- Chemically reactive, reactivity decreases slightly across the series.
- Uses: Production of alloy steels (e.g., Mischmetal - ~95% lanthanoid metal + ~5% Fe + traces of S, C, Ca, Al; used in magnesium-based alloys for bullets, shells, lighter flints), catalysts (petroleum cracking), phosphors in television screens, lasers.
- Actinoids (5f Series):
- Electronic Configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². More irregularities than lanthanoids due to comparable energies of 5f, 6d, and 7s orbitals.
- Oxidation States: Show a wider range of oxidation states than lanthanoids. +3 is common, but higher states like +4, +5, +6, +7 are shown by early actinoids (e.g., Th(+4), Pa(+5), U(+6), Np(+7)). This is because 5f, 6d, and 7s electrons can participate in bonding. The range decreases for later actinoids.
- Actinoid Contraction: Similar to lanthanoid contraction but greater magnitude from element to element due to poorer shielding by 5f electrons compared to 4f electrons.
- General Characteristics:
- All are radioactive. Later members have very short half-lives.
- Silvery appearance, but tarnish in air.
- Highly reactive metals, especially when finely divided.
- Form alloys.
- Most ions are coloured.
- Magnetic properties are more complex than lanthanoids.
- Comparison with Lanthanoids:
- Actinoids show a wider range of oxidation states.
- All actinoids are radioactive; only Promethium (Pm) among lanthanoids is radioactive.
- Actinoid contraction is greater.
- Actinoid compounds are generally more basic (less covalent) than corresponding lanthanoid compounds in the same oxidation state.
- Actinoids have a greater tendency to form complexes.
- Uses: Primarily nuclear fuel (U, Pu), nuclear weapons, atomic energy production. Thorium is used in cancer therapy and incandescent gas mantles.
Multiple Choice Questions (MCQs)
-
Which of the following elements is NOT generally considered a transition element, although studied with them?
(a) Fe
(b) Cu
(c) Zn
(d) Mn -
The similarity in atomic radii of Zirconium (Zr) and Hafnium (Hf) is best explained by:
(a) Actinoid Contraction
(b) Diagonal Relationship
(c) Lanthanoid Contraction
(d) Their presence in the same period -
The maximum oxidation state shown by Manganese (Mn, Z=25) is:
(a) +5
(b) +6
(c) +7
(d) +8 -
Which of the following ions is expected to be colourless in aqueous solution?
(a) Fe³⁺ (Z=26)
(b) Mn²⁺ (Z=25)
(c) Cr³⁺ (Z=24)
(d) Sc³⁺ (Z=21) -
The 'spin-only' magnetic moment (µ) of the Ti³⁺ ion (Z=22) in Bohr Magnetons (BM) is:
(a) √3
(b) √8
(c) √15
(d) 0 -
Interstitial compounds are formed when small atoms like H, C, or N are trapped in the crystal lattice of:
(a) Alkali metals
(b) Halogens
(c) Transition metals
(d) Noble gases -
In the reaction
Cr₂O₇²⁻ + 14 H⁺ + 6 Fe²⁺ → 2 Cr³⁺ + 7 H₂O + 6 Fe³⁺, the oxidizing agent is:
(a) H⁺
(b) Fe²⁺
(c) Cr³⁺
(d) Cr₂O₇²⁻ -
Potassium permanganate (KMnO₄) acts as an oxidizing agent in acidic, neutral, and alkaline media. In which medium is Mn reduced to MnO₂?
(a) Acidic medium
(b) Strongly alkaline medium
(c) Neutral or faintly alkaline medium
(d) It always forms Mn²⁺ -
The most common oxidation state exhibited by Lanthanoids is:
(a) +1
(b) +2
(c) +3
(d) +4 -
Which of the following statements about Actinoids is INCORRECT?
(a) All actinoids are radioactive.
(b) They exhibit a wider range of oxidation states than lanthanoids.
(c) Actinoid contraction is less pronounced than lanthanoid contraction.
(d) Early actinoids show higher oxidation states more frequently.
Answer Key:
- (c)
- (c)
- (c)
- (d)
- (a) [Ti (Z=22) is [Ar] 3d² 4s². Ti³⁺ is [Ar] 3d¹. n=1. µ = √1(1+2) = √3 BM]
- (c)
- (d)
- (c)
- (c)
- (c) [Actinoid contraction is greater than lanthanoid contraction]
Make sure you thoroughly understand the reasons behind these properties and trends, especially the electronic configurations and the consequences of lanthanoid contraction. These are frequently tested areas. Good luck with your preparation!