The d-and f-Block Elements

Unit 4: The d- and f-Block Elements

Introduction

The d-block elements occupy groups 3 to 12 of the periodic table, where electrons are progressively filled into the (n-1)d orbitals. They are also called transition metals. The f-block elements fill the (n-2)f orbitals and include the lanthanoids (Ce to Lu) and actinoids (Th to Lr), placed separately at the bottom of the periodic table.

---

The d-Block Elements (Transition Metals)

These span four series:
1st series: Sc (Z=21) to Zn (Z=30) — 3d series
2nd series: Y (Z=39) to Cd (Z=48) — 4d series
3rd series: La (Z=57) and Hf (Z=72) to Hg (Z=80) — 5d series
4th series: Ac (Z=89) and Rf (Z=104) to Cn (Z=112) — 6d series

Electronic configuration: Generally (n-1)d^1-10 ns^0-2
Exceptions: Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹ (extra stability of half-filled and fully-filled d orbitals).

---

General Properties of Transition Metals

1. Variable Oxidation States: Most transition metals exhibit multiple oxidation states because both (n-1)d and ns electrons can be used in bonding. Mn shows oxidation states from +2 to +7; Cr from +2 to +6.

2. Formation of Coloured Compounds: Transition metal ions absorb visible light due to d-d transitions (electron from lower d-level to higher). The complementary colour is observed. Zn2+ and Sc3+ are colourless (full or empty d-orbitals).

3. Magnetic Properties: Unpaired electrons make transition metal compounds paramagnetic. The magnetic moment = √(n(n+2)) BM where n = number of unpaired electrons.

4. Formation of Complex Compounds: The small, highly charged ions with available d-orbitals act as Lewis acids and readily form coordination compounds with ligands.

5. Catalytic Properties: Many transition metals and their compounds act as catalysts (Fe in Haber process, V2O5 in Contact process, Ni in hydrogenation, Pt in Ostwald process).

6. Alloy Formation: Transition metals form alloys with each other (e.g., steel, brass, bronze) because their atomic radii are similar.

7. High Melting Points: Strong metallic bonding involving d-electrons gives high melting points (W has the highest: 3410°C).

---

Important Trends in First Series

Atomic radii: Generally decrease from Sc to Cr, then nearly constant (d-d shielding becomes effective), then slight increase to Cu/Zn.
Ionisation enthalpy: Generally increases across the series (nuclear charge increases).
Colour of M2+ ions: Ti2+ violet; V2+ violet; Cr2+ blue; Mn2+ light pink; Fe2+ green; Co2+ pink; Ni2+ green; Cu2+ blue; Zn2+ colourless.
Reactivity: First series metals are more reactive than 2nd and 3rd series elements.

---

Important Compounds of Transition Metals

Potassium dichromate (K2Cr2O7):
Orange solid; strong oxidising agent in acidic medium
Cr is in +6 oxidation state
Cr2O7²- + 14H+ + 6e- → 2Cr3+ + 7H2O
Preparation: Chrome iron ore → Na2CrO4 (with NaOH + air) → Na2Cr2O7 (acidify) → K2Cr2O7

Potassium permanganate (KMnO4):
Purple/violet solid; powerful oxidising agent
Mn is in +7 oxidation state
In acidic medium: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
In neutral/basic medium: MnO4- + 2H2O + 3e- → MnO2 + 4OH-
Prepared from pyrolusite (MnO2): MnO2 → K2MnO4 (fusion with KOH) → KMnO4 (oxidation)

---

The f-Block Elements

Lanthanoids (Lanthanides): Ce (Z=58) to Lu (Z=71)
General configuration: [Xe] 4f^1-14 5d^0-1 6s²
Show lanthanoid contraction: Steady decrease in ionic/atomic radii across the series due to poor shielding by 4f electrons. Consequence: 4d and 5d elements of same group have nearly identical radii (e.g., Zr and Hf), making their separation difficult.
Predominantly show +3 oxidation state; some show +2 (Eu, Sm) or +4 (Ce, Tb, Pr).
Mostly silvery-white metals; form ionic compounds.
Used in alloys (misch metal), phosphors, superconductors, magnets.

Actinoids (Actinides): Th (Z=90) to Lr (Z=103)
General configuration: [Rn] 5f^1-14 6d^0-1 7s²
Show a wider range of oxidation states than lanthanoids.
All are radioactive; heavier elements (Md onwards) are synthetically made.
5f orbitals participate in bonding (unlike 4f).

---

Worked Examples

Example 1

Write the electronic configuration of Cr (Z=24) and Cu (Z=29).
Cr: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s² — half-filled d is more stable)
Cu: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s² — fully-filled d is more stable)

Example 2

Calculate magnetic moment of Fe3+ (Z=26).
Fe: [Ar] 3d⁶ 4s²; Fe3+: [Ar] 3d⁵ (5 unpaired electrons)
mu = √(5(5+2)) = √(35) = 5.92 BM

Example 3

Why does Mn show highest oxidation state of +7?
Mn: [Ar] 3d⁵ 4s². Total = 7 electrons in 3d and 4s; all can participate in bonding → maximum oxidation state = +7.

Example 4

In acidic KMnO4 titration, balance the equation when Fe2+ is oxidised by MnO4-.
Fe2+ → Fe3+ + e- (x5)
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O

Example 5

Why is Zn2+ colourless while Cu2+ is blue?
Zn2+: [Ar] 3d¹⁰ — fully filled d-orbitals; no d-d transitions possible → colourless.
Cu2+: [Ar] 3d⁹ — one unpaired electron allows d-d transition; absorbs red light → appears blue.

Example 6

What is lanthanoid contraction? What is its consequence?
Across lanthanoids, 4f electrons are added but they shield poorly, so effective nuclear charge increases, contracting the ionic radius. Consequence: Zr (Z=40) and Hf (Z=72) have almost identical atomic radii (~160 pm each), making their separation extremely difficult.

Example 7

In the Contact process for H2SO4 manufacture, V2O5 is the catalyst. Write the catalytic step.
2SO2 + O2 → 2SO3 (in the presence of V2O5)
V2O5 oxidises SO2 to SO3 (V⁵+ → V⁴+), then is re-oxidised by O2 (V⁴+ → V⁵+). This shows the variable oxidation state property of transition metals in catalysis.

---

Common mistakes

> - Writing the electronic configuration of Cr as [Ar] 3d⁴ 4s² instead of [Ar] 3d⁵ 4s¹. Remember the extra stability of half-filled d-orbitals.
> - Forgetting that Sc3+ and Zn2+ have empty and completely filled d-orbitals respectively, and hence form colourless compounds (no d-d transitions).
> - Confusing lanthanoid contraction consequences: it is the reason 4d and 5d elements in the same group have similar sizes, NOT 3d and 4d.

---

Summary

d-Block (transition) elements are characterised by variable oxidation states, coloured ions, catalytic activity, magnetic properties, and complex formation, all arising from partially filled d-orbitals. Cr and Cu have exceptional configurations. KMnO4 and K2Cr2O7 are important oxidising agents. f-Block elements (lanthanoids and actinoids) fill f-orbitals; lanthanoid contraction explains similarity in size of 4d and 5d elements. Actinoids are all radioactive.