Revision: d-block and f-block Elements Chemistry Science (English Medium) Class 12 CBSE

Elements having partially filled d-orbitals in ground state or in excited state are known as transition elements.

In the lanthanoids, the electrons are filling the 4f-subshell. On moving from left to right, the nuclear charge increases, and this increase is expected to be compensated by the increase in the magnitude of the shielding effect by the 4f-electrons. However, the f-electrons have very poor shielding effects. With an increasing atomic number in the lanthanoid series, there is a progressive decrease in the atomic as well as ionic radii of trivalent ions from La³⁺ to Lu^3+, and this is known as lanthanoid contraction.

A homogeneous mixture of two or more metals or a metal and a non-metal is called alloy.

The property of transition elements to exhibit more than one oxidation state is called variable oxidation state.

The electrode potential of a half-cell measured under standard conditions is called standard electrode potential.

The extra stability associated with parallel spin electrons in degenerate orbitals is called exchange energy.

The elements in which the last electron enters the (n–1)d subshell are called d-block elements.

The elements which have incompletely filled d-orbitals in their ground state or in any of their common oxidation states are called transition elements.

The elements in which electrons are progressively filled in the 4f or 5f orbitals are called inner transition elements.

The series of elements from cerium (Ce) to lutetium (Lu) in which 4f orbitals are progressively filled are called lanthanoids.

The series of elements from thorium (Th) to lawrencium (Lr) in which 5f orbitals are progressively filled are called actinoids.

The gradual decrease in atomic and ionic radii of lanthanoids with increasing atomic number is called lanthanoid contraction.

Compounds formed when small atoms like H, C or N occupy interstitial spaces in the crystal lattice of metals are called interstitial compounds.

The property of a substance to get attracted in a magnetic field due to the presence of unpaired electrons is called paramagnetism.

The property of a substance to get weakly repelled by a magnetic field due to the absence of unpaired electrons is called diamagnetism.

The property of a substance to be strongly attracted by a magnetic field and retain magnetism is called ferromagnetism.

Statement:
The standard electrode potential values of transition metals depend on ionisation enthalpy, enthalpy of atomisation and hydration enthalpy.

Explanation:

• More negative E° → stronger reducing agent.

• Mn and Zn show highly negative E° values.

• Cu shows positive E° value due to high ionisation enthalpy.

Statement:
Ionisation enthalpy generally increases across a transition series due to increase in effective nuclear charge.

Explanation:

• Increase in nuclear charge pulls electrons strongly.

• Variation is not smooth due to extra stability of half-filled and fully filled configurations.

• Removal of 4s electrons occurs before 3d electrons during ion formation.

Statement:
Transition elements exhibit variable oxidation states due to the comparable energies of ns and (n−1)d orbitals.

Explanation:

• Both ns and d electrons participate in bonding.

• Early elements show higher oxidation states.

• Middle elements show maximum oxidation states.

• Later elements prefer lower oxidation states.

Example:
Mn shows +2 to +7 oxidation states.

Statement:
The magnetic behaviour of transition metal ions depends on the number of unpaired electrons present in their d-orbitals.

Explanation:

• Presence of unpaired electrons → Paramagnetic

• Absence of unpaired electrons → Diamagnetic

• Magnetic moment increases with increase in number of unpaired electrons

• Given by formula: \[\mu=\sqrt{n(n+2)}\]

Statement:
Transition metal ions are coloured due to d–d electronic transitions in the presence of ligands.

Explanation:

• Absorption of visible light promotes electron from lower to higher d-orbital.

• Colour observed is complementary to absorbed light.

• d⁰ and d¹⁰ configurations are colourless (e.g., Zn²⁺, Sc³⁺).

Statement:
Transition metals act as catalysts due to their variable oxidation states and ability to form intermediate complexes.

Explanation:

• They provide alternate reaction pathway.

• Lower activation energy.

• Surface adsorption of reactants.

Examples:

• Fe in Haber process
• V₂O₅ in Contact process
• Ni in hydrogenation

Statement:
The most stable oxidation state of lanthanoids is +3 due to the stable 4f electronic configuration.

Explanation:

• Some show +2 and +4 states due to extra stability of empty, half-filled or fully filled f-orbitals.

• Ce⁴⁺ and Eu²⁺ are important examples.