d-Block and f-Block Elements

d-Block elements = transition metals. Have partly filled d-orbitals in atom or in at least one of their common ions.

Three series:

• 3d series (Sc → Zn): Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn.
• 4d series (Y → Cd).
• 5d series (La/Hf → Hg).
• 6d series (Ac/Rf onwards, mostly synthetic).

Strict definition: Zn, Cd, Hg have full d¹⁰ — not transition metals by definition (the IUPAC restriction). But often included in NCERT.

Characteristic properties of transition metals:

1. Variable oxidation states. Multiple stable oxidation states because (n-1)d and ns electrons have similar energies.

Maximum OS in the middle of the series (Mn +7).

2. Coloured ions.

d-d transitions in the visible spectrum cause color.

• Ti³⁺ (d¹): purple
• V³⁺ (d²): green
• Cr³⁺ (d³): violet
• Mn²⁺ (d⁵): faint pink (forbidden transition)
• Fe³⁺ (d⁵): yellow-brown
• Co²⁺ (d⁷): pink
• Ni²⁺ (d⁸): green
• Cu²⁺ (d⁹): blue
• Zn²⁺ (d¹⁰): colorless (no d-d transitions)
• Sc³⁺ (d⁰): colorless

So d⁰ and d¹⁰ ions are colourless.

3. Paramagnetism. Unpaired d-electrons → magnetic moment.

μ = √(n(n+2)) BM, where n = unpaired electrons.

• Sc³⁺ (d⁰): 0 unpaired → diamagnetic
• Cu²⁺ (d⁹): 1 unpaired → μ ≈ 1.73 BM
• Mn²⁺ (d⁵): 5 unpaired → μ ≈ 5.92 BM (maximum)

4. Catalytic activity. Reasons: variable oxidation states + ability to form complexes.

Famous catalysts:

• Iron (Fe) — Haber process (N₂ + 3 H₂ → 2 NH₃).
• V₂O₅ — Contact process (SO₂ → SO₃).
• Ni — hydrogenation of oils (margarine production).
• TiCl₃/Al(C₂H₅)₃ — Ziegler-Natta (polythene).
• Pt — catalytic converters (CO + NO → CO₂ + N₂).
• MnO₂ — decomposition of KClO₃, H₂O₂.

5. Formation of complex compounds. Empty d-orbitals can accept lone pairs from ligands. (See coordination compounds.)

6. Alloy formation. Similar atomic sizes → easy substitution. Steel, brass (Cu+Zn), bronze (Cu+Sn).

7. Interstitial compounds. Small atoms (H, C, N) lodge in interstices of the metal lattice. Steel = Fe + C interstitial.

KMnO₄ and K₂Cr₂O₇ — two important oxidizing agents:

KMnO₄ (potassium permanganate, purple):

• In acidic medium: Mn⁷⁺ → Mn²⁺ (5-electron change). Strong oxidizer.
• In neutral / weak basic: Mn⁷⁺ → Mn⁴⁺ (MnO₂, brown precipitate).
• In strong basic: Mn⁷⁺ → Mn⁶⁺ (MnO₄²⁻, green).

K₂Cr₂O₇ (potassium dichromate, orange):

• Cr⁶⁺ → Cr³⁺ in acidic medium (3-electron change per Cr; 6 per dichromate).
• Used as primary standard in volumetric analysis (high purity, stable).

LANTHANIDE CONTRACTION

The 4f electrons of lanthanides shield poorly → effective nuclear charge increases as you go across the lanthanide series → atomic/ionic radius decreases.

Consequence: the 5d transition metals (Hf, Ta, W, ...) end up with nearly identical sizes to their 4d counterparts (Zr, Nb, Mo, ...). This makes Zr/Hf, Nb/Ta chemically very similar — hard to separate.

f-Block elements = the 14 lanthanides (Ce → Lu) + 14 actinides (Th → Lr). Their differentiating electron enters the (n−2)f orbital.

LANTHANIDES (4f series, Z = 58 to 71)

14 elements: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

(La is sometimes included as part of the lanthanide series, sometimes as the prefix.)

Common oxidation state: +3 (lose one 6s + two more electrons from 5d/4f).

Some exhibit +2 or +4:

• Eu²⁺ (4f⁷ — half-filled stability).
• Yb²⁺ (4f¹⁴ — fully filled).
• Ce⁴⁺ (4f⁰ — empty, like Xe).
• Tb⁴⁺ (4f⁷ — half-filled).

LANTHANIDE CONTRACTION — most important fact.

Going from Ce to Lu, atomic and ionic radii decrease steadily.

Cause: 4f electrons shield poorly → effective nuclear charge increases → radius shrinks.

Total contraction across the series: ~17 pm.

Consequences:

1. 5d transition metals have nearly identical sizes to corresponding 4d metals. For example, Zr (4d, 160 pm) ≈ Hf (5d, 159 pm). This makes Zr/Hf chemically very similar and hard to separate.
2. Same for Nb/Ta, Mo/W.
3. Densities of 5d elements are very high (heavy atoms packed into small volumes): Os and Ir are the densest (~22.6 g/cm³).
4. Basicity of Ln(OH)₃ decreases across the lanthanide series.

Colour and magnetism:

Most Ln³⁺ ions are coloured (4f-4f transitions). Some are colourless: La³⁺ (4f⁰), Lu³⁺ (4f¹⁴), Yb²⁺ (4f¹⁴), Ce⁴⁺ (4f⁰), Y³⁺.

Magnetic moment of lanthanides differs from transition metals — must include orbital + spin contribution (LS coupling).
Formula: μ = g√(J(J+1)) BM, where J = L ± S.

Uses of lanthanides:

• Misch metal: mix of Ce, La, Nd, Pr + iron — used in lighter flints, alloys.
• Neodymium: strong permanent magnets (Nd-Fe-B). In EV motors, hard disks, headphones.
• Europium, terbium: phosphors in LEDs and TV screens (red, green).
• Erbium: fiber-optic amplifiers.
• Gadolinium: MRI contrast agents.
• Cerium oxide: catalytic converters, glass polishing.

Strategic importance: rare earth elements are critical for modern tech — but China controls ~70% of global supply, leading to supply chain anxieties.

ACTINIDES (5f series, Z = 90 to 103)

14 elements: Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr.

All actinides are radioactive.

Natural actinides: Th, U (rest only in trace amounts or synthesized).
Synthetic transuranic actinides: Np onwards.

Variable oxidation states much more than lanthanides (because 5f, 6d, 7s have similar energies). Examples:

• U: +3, +4, +5, +6 (most stable +6 as UO₂²⁺).
• Pu: +3 to +7.
• Th: only +4.

Actinide contraction: similar to lanthanide contraction. 5f electrons shield poorly. Radii decrease across series.

Uses of actinides (mostly nuclear):

Uranium (U):

• Natural U: 99.27% U-238 (fertile) + 0.72% U-235 (fissile) + traces of U-234.
• U-235: fuel in light-water reactors and atomic bombs (after enrichment to 3-5% for power, ~90% for weapons).
• U-238: absorbs neutrons to become Pu-239 (in breeder reactors).

Thorium (Th):

• Th-232 is fertile (becomes U-233 after neutron capture).
• India has ~25% of world's thorium reserves (Kerala monazite sands).
• India's 3-stage nuclear program aims to exploit thorium in advanced heavy water reactors.

Plutonium (Pu):

• Pu-239: produced from U-238 in reactors. Fissile. Used in fast breeder reactors and weapons.

Americium (Am):

• Am-241: used in smoke detectors (emits α particles).

Heavy actinides (Bk and beyond): synthesized in cyclotrons; very short half-lives; primarily research interest.