d and f Block Elements

Definition

Definition 1 (IUPAC Definition): A transition element is defined as an element that has an incompletely filled d subshell either in its neutral ground state or in any of its common oxidation states.

Definition 2 (Standard Textbook Definition by J.D. Lee): Transition metals are those elements which partially fill their d orbitals, giving them characteristic properties such as variable oxidation states, coloured ions, and catalytic activity.

Explanation

• They lie in groups 3 through 12 of the periodic table, forming a bridge between the s block and p block elements.
• They span four distinct periods: 3d series (Sc to Zn), 4d series (Y to Cd), 5d series (La to Hg, excluding Lu), and the incomplete 6d series.
• Note on Group 12 Elements: Zinc, cadmium, and mercury have completely filled d orbitals ($d^{10}$ configuration) in their ground state and common oxidation states. Hence, they are traditionally studied with transition elements due to chemical similarities, but strictly speaking, are not true transition elements.

Examples

• The Iron Bridges We Cross: Steel used in modern infrastructure is an alloy of iron, a classic d block transition element.
• Gold Jewellery: Gold (5d series) is prized for its inertness, lustre, and beauty stemming from its d orbital electronic structure.

Definition

The general electronic configuration of the d block elements is represented as (n-1)d^{1-10}ns^{1-2}, where $n$ represents the outermost principal energy shell, and $(n-1)$ stands for the penultimate shell.

Explanation

• As we move across a period, electrons are successively added to the inner $(n-1)d$ orbitals rather than the outermost $ns$ orbital.
• Because $(n-1)d$ and $ns$ orbitals are close in energy, electrons can easily be shared or promoted, resulting in multiple oxidation states.

Examples

• Scandium (Sc, Z=21): $[Ar] 3d^1 4s^2$
• Chromium (Cr, Z=24): $[Ar] 3d^5 4s^1$ (Half-filled stability rule)
• Copper (Cu, Z=29): $[Ar] 3d^{10} 4s^1$ (Completely filled stability rule)

Definition

Transition elements are metals occurring abundantly in the Earth's crust as oxides, sulphides, or carbonates, characterized by variable oxidation states, coloured ions, paramagnetism, and catalytic properties.

Explanation

• Atomic and Ionic Radii: Decrease initially, remain constant in the middle, and increase slightly towards the end due to a balance between nuclear charge and electron-electron repulsion.
• Ionisation Enthalpies: High values, lying intermediate between s-block and p-block elements.
• Colour of Ions: Caused by d-d electronic transitions absorbing visible light.
• Magnetic Properties: Paramagnetic due to unpaired electrons.
• Catalytic Properties: Excellent catalysts because of multiple oxidation states and surface-binding abilities.

Comparison Table

Definition

Powerful oxidizing agents widely used in chemical laboratories and industrial chemistry derived from chromite and pyrolusite ores respectively.

Explanation

• Potassium Dichromate ($K_2Cr_2O_7$): Prepared via chromite ore fusion with sodium carbonate, acidification to sodium dichromate, and double decomposition with KCl. Used in breathalysers.
• Potassium Permanganate ($KMnO_4$): Prepared via fusion of pyrolusite ($MnO_2$) with KOH and oxidising agent, followed by electrolytic oxidation of potassium manganate. Used in water purification.

Examples

Acidic Dichromate Half-Reaction:
Cr2O7^2- + 14H^+ + 6e^- -> 2Cr^3+ + 7H2O

Acidic Permanganate Half-Reaction:
MnO4^- + 8H^+ + 5e^- -> Mn^2+ + 4H2O
        

Definition

Elements where the last electron enters the f orbitals, split into the lanthanoid (4f series) and actinoid (5f series) inner transition series.

Explanation

• Lanthanoids: Elements from Ce (58) to Lu (71). General configuration: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. Exhibit prominent lanthanoid contraction due to poor shielding of nuclear charge by 4f electrons.
• Actinoids: Elements from Th (90) to Lr (103). General configuration: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. All actinoids are radioactive and show a greater range of oxidation states.

Examples

• Uranium-235: An actinoid utilized as nuclear fuel in power generation.
• Gadolinium: A lanthanoid used in MRI contrast agents.

Definition

Application of magnetic moment formulas to determine unpaired electrons in transition metal ions.

Numerical Illustration

Problem: Calculate the spin-only magnetic moment of a divalent metal ion ($M^{2+}$) with atomic number 25 in aqueous solution.

Step 1: Identify element (Atomic Number 25 = Manganese, Mn).
Step 2: Ground state configuration: [Ar] 3d^5 4s^2.
Step 3: M^2+ ion configuration: [Ar] 3d^5.
Step 4: Unpaired electrons (n) = 5.
Step 5: Apply formula: μ = sqrt(n(n+2)) BM.
Step 6: Calculate: μ = sqrt(5(7)) = sqrt(35) ≈ 5.92 BM.
        

Formulas

• Spin-only Magnetic Moment: $\mu = \sqrt{n(n+2)}$ BM

Key Equations

Oxidation of I^- by Acidified Dichromate:
Cr2O7^2- + 14H^+ + 6I^- -> 2Cr^3+ + 3I2 + 7H2O

Oxidation of Fe^2+ by Acidified Permanganate:
MnO4^- + 8H^+ + 5Fe^2+ -> Mn^2+ + 4H2O + 5Fe^3+
        

Explanation

• Catalytic Converters: Platinum, palladium, and rhodium convert toxic exhaust gases into safer emissions.
• MRI Contrast Agents: Gadolinium complexes improve medical imaging clarity.
• Nuclear Reactors: Uranium and plutonium power commercial nuclear fission facilities.

Case Study: The Catalyst That Powered Modern Agriculture

The Haber-Bosch process synthesized ammonia from atmospheric nitrogen and hydrogen gas using a finely divided metallic iron catalyst promoted with potassium and aluminium oxides. Iron's vacant d orbitals lower activation energy, enabling global food production for eight billion people.

1. Which of the following elements is NOT a transition element in terms of chemical definition?
   A) Iron (Fe) | B) Zinc (Zn) | C) Copper (Cu) | D) Titanium (Ti)
   Answer: B
   Explanation: Zinc has a completely filled $3d^{10}$ configuration in ground and common $+2$ states.
2. The general electronic configuration of d block elements is represented as:
   A) $ns^{1-2} np^{1-6}$ | B) $(n-1)d^{1-10} ns^{1-2}$ | C) $(n-2)f^{1-14} ns^{2}$ | D) $ns^{2} np^{6} (n-1)d^{10}$
   Answer: B
3. The cause of lanthanoid contraction is:
   A) Effective shielding | B) Poor shielding of nuclear charge by 4f electrons | C) Increase in atomic number | D) Decrease in nuclear charge
   Answer: B
4. What is the spin-only magnetic moment of a $Ti^{3+}$ ion (Atomic number of Ti = 22)?
   A) $1.73$ BM | B) $2.83$ BM | C) $3.87$ BM | D) $0.00$ BM
   Answer: A
   Explanation: $Ti^{3+}$ has $n=1$, giving $\mu = \sqrt{3} \approx 1.73$ BM.
5. Which of the following ions is coloured in aqueous solution due to d-d transitions?
   A) $Sc^{3+}$ | B) $Zn^{2+}$ | C) $Ti^{3+}$ | D) $Cu^+$
   Answer: C
6. Potassium dichromate in acidic medium acts as a:
   A) Strong reducing agent | B) Strong oxidising agent | C) Dehydrating agent | D) Bleaching agent
   Answer: B
7. Which transition series contains the element gold (Au)?
   A) 3d series | B) 4d series | C) 5d series | D) 6d series
   Answer: C
8. Mischmetal is an alloy consisting chiefly of a lanthanoid metal and:
   A) Iron | B) Copper | C) Carbon | D) Aluminium
   Answer: A
9. Why do transition elements exhibit variable oxidation states?
   A) Tightly bound outer electrons | B) Small energy difference between $(n-1)d$ and $ns$ orbitals | C) Noble gas configurations | D) No d electrons
   Answer: B
10. Which actinoid is commonly used as nuclear fuel in power reactors?
    A) Thorium | B) Uranium | C) Neptunium | D) Lawrencium
    Answer: B

1. Q: Why are compounds of transition elements generally coloured?
   Answer: Due to unpaired d electrons permitting d-d electronic transitions upon absorbing visible light.
2. Q: Explain why transition metals and their compounds act as good catalysts.
   Answer: Due to variable oxidation states and active surface area facilitating reactant adsorption and bond-breaking.
3. Q: State two consequences of lanthanoid contraction.
   Answer: 1. Almost identical radii/properties of 4d and 5d elements (e.g., Zr and Hf). 2. Steady decrease in hydroxide basic strength.
4. Q: Why does zinc have the lowest enthalpy of atomisation among 3d transition series elements?
   Answer: Zinc has a fully filled $3d^{10} 4s^2$ configuration with no unpaired electrons, resulting in very weak metallic bonding.
5. Q: Copper has a positive electrode potential ($+0.34$ V), unlike most other first-row transition metals. Why?
   Answer: The high energy required to transform solid copper into gaseous atoms and ions is not balanced by its hydration enthalpy.
6. Q: Write the chemical equations involved in the preparation of potassium permanganate from pyrolusite ore.
   Answer: 1) $2MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O$
   2) $3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$
7. Q: Out of $Cr^{2+}$ and $Mn^{3+}$, which is a stronger reducing agent and why?
   Answer: $Cr^{2+}$ because oxidation to $Cr^{3+}$ yields a stable half-filled $t_{2g}$ ($3d^3$) configuration.
8. Q: Why do transition elements form coordination complexes easily?
   Answer: Due to small, highly charged metal ions possessing vacant d-orbitals to accept electron pairs from ligands.
9. Q: What is mischmetal? Give its primary use.
   Answer: An alloy of ~95% lanthanoid metal and 5% iron, used in lighter flints and tracer bullets.
10. Q: Explain why the highest oxidation state of a transition metal is often exhibited in its oxides or fluorides.
    Answer: Oxygen and fluorine are small, highly electronegative atoms that stabilize high oxidation states through strong bonds.

• d-block elements occupy groups 3-12; f-block elements form the lanthanoid and actinoid series.
• Zinc, cadmium, and mercury are d-block but not true transition elements due to $d^{10}$ configurations.
• Lanthanoid contraction causes close atomic radii similarities between 4d and 5d series elements.

Mnemonic for 3d Series: Science Ti-teacher Vipin Cried Mn Feevar Co Ni Cu Zoon! (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn)