Chapter 4: d & f Block Elements

What are Transition Elements?

Elements having incompletely filled d-orbitals in their ground state or in any of their common oxidation states. Zinc, Cadmium, and Mercury (Zn, Cd, Hg) have fully filled d-orbitals (d¹⁰), hence they are not considered typical transition elements.

General Electronic Configuration

(n-1)d^1-10 ns^1-2

Important Exceptions

• Chromium (Cr, Z=24): [Ar] 3d⁵ 4s¹ (Half-filled stability)
• Copper (Cu, Z=29): [Ar] 3d¹⁰ 4s¹ (Fully-filled stability)

Enthalpy of Atomization

Transition metals have high enthalpies of atomization because of strong metallic bonding due to the involvement of a large number of unpaired electrons in the (n-1)d orbitals.

Atomic and Ionic Radii

• Radii decrease with increase in atomic number in a given series.
• Lanthanoid Contraction: The almost identical radii of Zr (160 pm) and Hf (159 pm) is due to the poor shielding effect of 4f electrons, which increases effective nuclear charge.

Ionization Enthalpies

Generally increases from left to right along a series. However, the increase is not as steep as in the case of s and p block elements.

Variable Oxidation States

Transition elements show variable oxidation states due to the participation of both ns and (n-1)d electrons (because their energy levels are very close).

• Scandium (Sc): Shows only +3 oxidation state.
• Manganese (Mn): Shows maximum number of oxidation states (+2 to +7) in the 3d series.
• Osmium (Os) & Ruthenium (Ru): Show the highest oxidation state of +8.

Magnetic Properties

Due to the presence of unpaired electrons, most transition metal ions are Paramagnetic.

Spin-Only Magnetic Moment (μ):

μ = √(n(n+2)) B.M.

Where 'n' is the number of unpaired electrons and B.M. is Bohr Magneton.

Formation of Coloured Ions

Most of the transition metal compounds are coloured in solid form or in solution. This is due to d-d transition. When visible light falls, an electron absorbs energy and jumps from a lower energy d-orbital to a higher energy d-orbital. Ions with d⁰ or d¹⁰ configuration (like Zn²⁺, Ti⁴⁺) are colourless.

Catalytic Properties

They act as excellent catalysts because:

• They can show multiple oxidation states.
• They provide a large surface area for adsorption of reactant gases.
• Example: V₂O₅ in Contact Process, Fe in Haber's Process.

Interstitial Compounds & Alloys

• Interstitial Compounds: Formed when small atoms like H, C, or N are trapped inside the crystal lattices of metals. They are very hard and have high melting points.
• Alloys: Formed easily because transition metals have similar atomic radii (difference is less than 15%), allowing them to replace each other in the crystal lattice.

Potassium Dichromate (K₂Cr₂O₇)

Acts as a strong oxidizing agent in acidic medium.

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

• Oxidizes Iodide (I⁻) to Iodine (I₂).
• Oxidizes Fe²⁺ to Fe³⁺.
• Chromate (yellow) and Dichromate (orange) are interconvertible depending on pH.

Potassium Permanganate (KMnO₄)

Intense purple colour. Acts as a very strong oxidizing agent in acidic, neutral, and alkaline mediums.

In Acidic Medium:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

• Oxidizes Oxalate (C₂O₄²⁻) to CO₂.
• Oxidizes Fe²⁺ to Fe³⁺.

Lanthanoids (4f series)

• General configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
• Most common oxidation state is +3 (Though Ce shows +4 and Eu shows +2).
• Lanthanoid Contraction: Steady decrease in atomic/ionic radii from La to Lu due to poor shielding of 4f electrons.
• Used in making Mischmetal (an alloy used in bullets and lighter flints).

Actinoids (5f series)

• General configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
• All are radioactive. Elements after Uranium are called Transuranic elements.
• Show a large number of oxidation states because 5f, 6d, and 7s levels are of comparable energies.
• Actinoid contraction is greater from element to element than lanthanoid contraction due to even poorer shielding by 5f electrons.