Chapter 4: d & f Block

What are Transition Elements?

Elements having incompletely filled d-orbitals in their ground state or in any of their common oxidation states.

• Exceptions: Zinc (Zn), Cadmium (Cd), and Mercury (Hg) have fully filled d-orbitals (d¹⁰) in ground and oxidation states, hence they are not considered typical transition elements.

General Electronic Configuration

• 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. Why? 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 generally decrease from left to right along a period due to increasing effective nuclear charge.
• Lanthanoid Contraction: The almost identical atomic radii of Zr (160 pm) and Hf (159 pm) in the 4d and 5d series respectively. It occurs due to the poor shielding effect of 4f electrons, which heavily increases the effective nuclear charge pulling the valence shell inwards.

Variable Oxidation States

They show variable oxidation states because the energy difference between (n-1)d and ns orbitals is very small, allowing electrons from both to participate in bonding.

• Scandium (Sc): Shows only +3 oxidation state.
• Manganese (Mn): Shows maximum number of oxidation states (+2 to +7).
• Osmium (Os): Shows the highest oxidation state of +8.

Magnetic Properties & Colour

• Paramagnetism: Due to the presence of unpaired electrons.
• Spin-Only Magnetic Moment Formula:
  μ = √(n(n+2)) B.M. (Where 'n' = number of unpaired electrons).
• Colour: Due to d-d transition. Electrons absorb specific visible light and jump to higher d-orbitals. (Ions with d⁰ or d¹⁰ like Zn²⁺, Ti⁴⁺ are colourless).

Catalysis & Alloys

• Catalysts: They act as good catalysts because they can adopt multiple oxidation states and provide large surface area (e.g., V₂O₅ in Contact Process).
• Interstitial Compounds: Small non-metals (H, C, N) trapped in crystal lattices. They are extremely hard with very high melting points.

Preparation & Properties

Prepared from Chromite ore (FeCr₂O₄). It acts as a powerful oxidizing agent in acidic medium.

• Oxidizes Iodide (I⁻) to Iodine (I₂).
• Oxidizes Ferrous (Fe²⁺) to Ferric (Fe³⁺).
• Oxidizes H₂S to elemental Sulphur (S).

Chromate-Dichromate Equilibrium

They are interconvertible depending on the pH of the solution.

• In Acidic medium (pH < 7): Yellow Chromate (CrO₄²⁻) turns into Orange Dichromate (Cr₂O₇²⁻).
• In Basic medium (pH > 7): Orange turns back to Yellow.

Preparation & Oxidizing Action

Prepared from Pyrolusite ore (MnO₂). Dark purple crystals. It acts as an oxidizing agent in acidic, neutral, and alkaline mediums.

In Acidic Medium (Strongest action):

• Oxidizes Oxalate (C₂O₄²⁻) to CO₂.
• Oxidizes Fe²⁺ to Fe³⁺.
• Oxidizes Nitrite (NO₂⁻) to Nitrate (NO₃⁻).

Lanthanoids (4f series)

• Most common oxidation state is +3.
• Cerium (Ce): Shows +4 state (acts as good analytical oxidizing agent).
• Mischmetal: An alloy of lanthanoid metals (~95%) with Iron (~5%) and traces of S, C, Ca, Al. Used in bullets and lighter flints.

Actinoids (5f series)

• All actinoids are radioactive. Elements after Uranium (Z=92) are called Transuranic elements.
• Show a larger number of oxidation states than lanthanoids because 5f, 6d, and 7s levels are of comparable energies.
• Actinoid Contraction: It is greater from element to element than lanthanoid contraction due to even poorer shielding by 5f electrons compared to 4f electrons.