Quick Revision Notes & Formula Sheet

Class 12 Chemistry: The d- and f-Block Elements

Madhya Pradesh Board (MPBSE)

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Introduction to d-Block Elements (Transition Elements)

• Definition: Elements in which the last electron enters the penultimate $(n-1)d$ orbitals. Their general electronic configuration is $(n-1)d^{1-10} ns^{1-2}$.
• Transition Element Criterion: An element which has partially filled $(n-1)d$ orbitals in its ground state or in any of its oxidation states (e.g., $Zn$, $Cd$, and $Hg$ have $d^{10}$ configuration in ground and common oxidation states, so they are transition metals by position but strictly speaking not transition elements).

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### Key Series of d-Block Elements

1. 3d-Series (Scandium to Zinc): $[Ar] 3d^{1-10} 4s^{1-2}$ (Period 4)
2. 4d-Series (Yttrium to Cadmium): $[Kr] 4d^{1-10} 5s^{1-2}$ (Period 5)
3. 5d-Series (Lanthanum, Hafnium to Mercury): $[Xe] 4f^{14} 5d^{1-10} 6s^{1-2}$ (Period 6)
4. 6d-Series (Actinium, Rutherfordium to Copernicium): Incomplete series.

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### General Properties of d-Block Elements

1. Atomic and Ionic Sizes

• Across a period, radii decrease initially, become nearly constant (due to balancing of shielding effect and nuclear charge), and increase slightly towards the end (due to electron-electron repulsion).
• Down a group, atomic radii increase from 3d to 4d series, but 4d and 5d series have nearly identical radii due to the Lanthanide Contraction.

2. Ionization Enthalpies

• High ionization enthalpies compared to s-block elements.
• Increases irregularly across a period due to addition of electrons and increasing nuclear charge.

3. Oxidation States

• Exhibit variable oxidation states because the energy difference between $(n-1)d$ and $ns$ orbitals is very small.
• Minimum oxidation state = Number of $ns$ electrons.
• Maximum oxidation state = Number of $ns$ electrons + Number of unpaired $(n-1)d$ electrons.
• Manganese (Mn) shows the maximum oxidation states in 3d series from $+2$ to $+7$.

4. Magnetic Properties

• Most transition metal ions are paramagnetic (attracted by magnetic field) due to the presence of unpaired electrons.
• Formula for Spin-Only Magnetic Moment ($\mu$): $$\mu = \sqrt{n(n + 2)} \text{ BM}$$ (where $n$ = number of unpaired electrons, and $BM$ = Bohr Magneton)
• If $n = 0$, the substance is diamagnetic (repelled by magnetic field).

5. Colour of Ions and Compounds

• Most of the compounds/ions of d-block elements are coloured in solid state or in aqueous solution.
• Due to d-d transition: Unpaired electrons in lower d-orbitals absorb visible light and jump to higher d-orbitals.

6. Catalytic Properties

• Transition metals act as good catalysts due to:
  • Variable oxidation states.
  • Ability to form reaction intermediates.
  • Large surface area and free valencies.
  • Examples: $V_2O_5$ (Contact Process), $Fe$ (Haber's Process), $Ni$ (Hydrogenation).

7. Interstitial Compounds

• Formed when small atoms like $H, C, N, B$ are trapped inside the crystal lattices of transition metals.
• Properties: Hard, high melting points, retain metallic conductivity, chemically inert.

8. Alloy Formation

• Due to similar atomic radii, atoms of one metal can easily replace atoms of another metal in the crystal lattice, forming alloys (e.g., Brass, Bronze, Steel).

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### Important Compounds of d-Block Elements

1. Potassium Dichromate ($K_2Cr_2O_7$)

• Preparation:
  1. Fusion of Chromite ore ($FeO \cdot Cr_2O_3$) with $Na_2CO_3$ in presence of air: $$4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$$
  2. Conversion of sodium chromate to dichromate: $$2Na_2CrO_4 + 2H^+ \rightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O$$
  3. Treatment of sodium dichromate with $KCl$: $$Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl$$
• Structure: Consists of two tetrahedral $CrO_4$ sharing one corner ($Cr-O-Cr$ bond angle is $126^\circ$).
• Oxidizing Property (in acidic medium): $$Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$$
  • Oxidizes $I^-$ to $I_2$, $Fe^{2+}$ to $Fe^{3+}$, $H_2S$ to $S$.

2. Potassium Permanganate ($KMnO_4$)

• Preparation: Fusion of Pyrolusite ore ($MnO_2$) with $KOH$ in presence of $O_2$: $$2MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O$$ Electrolytic oxidation of manganate to permanganate: $$MnO_4^{2-} \xrightarrow{\text{Oxidation}} MnO_4^- + e^-$$
• Oxidizing Property:
  • In Acidic Medium: $$MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$$
  • In Neutral/Faintly Alkaline Medium: $$MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 + 4OH^-$$

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### f-Block Elements (Inner Transition Elements)

• Definition: Elements in which the last electron enters the ante-penultimate $(n-2)f$ orbitals.
• Divided into two series:
  1. Lanthanoids (4f-series): Elements from Cerium ($Ce, 58$) to Lutetium ($Lu, 71$). General configuration: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$.
  2. Actinoids (5f-series): Elements from Thorium ($Th, 90$) to Lawrencium ($Lr, 103$). General configuration: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. All are radioactive.

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### Key Terms & Concepts in f-Block

1. Lanthanoid Contraction

• Definition: The steady decrease in atomic and ionic radii ($M^{3+}$ ions) of lanthanoids with increasing atomic number is called lanthanoid contraction.
• Cause: Imperfect shielding effect of 4f electrons. The nuclear charge increases by one unit at each step, but the 4f electrons are very poor at shielding the outer electrons from the nucleus, causing the valence shell to pull inward.
• Consequences:
  • Similarity in size of elements of 4d and 5d series (e.g., $Zr$ and $Hf$ have almost identical radii and are called chemical twins).
  • Difficulty in the separation of lanthanoids.
  • Variation in basic strength of hydroxides (basic strength decreases from $La(OH)_3$ to $Lu(OH)_3$).

2. Differences between Lanthanoids and Actinoids

Property	Lanthanoids (4f)	Actinoids (5f)
Radioactivity	Except Promethium ($Pm$), they are non-radioactive.	All are radioactive.
Oxidation States	$+3$ (common), $+2$, and $+4$ (due to stable $f^0, f^7, f^{14}$).	$+3, +4, +5, +6, +7$ (greater range due to small energy gap between $5f, 6d,$ and $7s$).
Binding Energy	4f orbitals have lower binding energy.	5f orbitals have higher binding energy.
Complex Formation	Less tendency to form complexes.	Greater tendency to form complexes.