The d- and f-Block Elements — Class 12 MCQs with Answers

The d- and f-Block Elements — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   d-block elements have last electron in:

   Ad-orbitalBs-orbitalCp-orbitalDf-orbital
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   Answer: d-orbital

   Explanation: The d-block gets its name because the last electron fills an (n−1)d orbital, e.g. Sc = [Ar]3d¹4s² — contrast s-block (last electron in ns) and p-block (np).

2. Q2 of 6Easy

   Lanthanide contraction refers to the:

   ADecrease in 3d seriesBSudden expansion of f-orbitalsCIncrease in radii of actinidesDSteady decrease in atomic and ionic radii across the 4f series
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   Answer: Steady decrease in atomic and ionic radii across the 4f series

   Explanation: Lanthanide contraction is the steady, cumulative decrease in atomic and ionic radii from La to Lu across the 4f series, caused by the poor shielding that 4f electrons provide for each added proton.

3. Q3 of 6Medium

   Transition metals show:

   AVariable oxidation statesBFixed +1 onlyCOnly +2DNone
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   Answer: Variable oxidation states

   Explanation: Because the (n−1)d and ns electrons lie close in energy, transition metals can lose different numbers of electrons, giving variable oxidation states — e.g. Mn ranges from +2 to +7, unlike the fixed +1 of alkali metals.

4. Q4 of 6Medium

   Assertion (A): The actinoid contraction is greater from element to element than the lanthanoid contraction. Reason (R): All the actinoids are radioactive elements.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: the size decrease per element is greater in the actinoids because 5f electrons shield one another poorly. R is true: all actinoids are radioactive. But radioactivity is a nuclear property and has nothing to do with how electrons shield one another, so R does not explain A.

5. Q5 of 6Hard

   Why are many transition metals good catalysts?

   AColoured ionsBLow densityCSoft textureDVariable oxidation states + ability to adsorb reactants
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   Answer: Variable oxidation states + ability to adsorb reactants

   Explanation: Transition metals catalyse reactions because their variable oxidation states let them form unstable intermediates, and their surfaces adsorb reactants, weakening bonds — colour and low density play no role in catalysis.

6. Q6 of 6Hard

   Assertion (A): Ce(IV) is of no use as an analytical reagent in aqueous solution. Reason (R): E° for the Ce⁴⁺/Ce³⁺ couple is +1.74 V, which suggests that Ce⁴⁺ can oxidise water.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: Ce(IV) is a good analytical reagent. R is true: the high E° means Ce⁴⁺ could oxidise water, but this reaction is very slow, so in practice Ce⁴⁺ solutions can be used as an oxidant. The rate, not just E°, decides its usefulness.

The d- and f-Block Elements notes: key concepts (Class 12 Chemistry)

The d- and f-Block Elements covers the transition and inner-transition metals: their (n−1)d ns configurations, variable oxidation states, coloured and paramagnetic ions, catalytic behaviour, alloy and complex formation, the lanthanoid contraction, and the preparation and oxidising action of K₂Cr₂O₇ and KMnO₄.

Transition (d-block) elements

Elements with an incompletely filled d-subshell in the atom or a common ion; general configuration (n−1)d¹⁻¹⁰ ns⁰⁻². Groups 3–12 of periods 4–7.

Variable oxidation states

The (n−1)d and ns electrons have similar energies, so both take part in bonding, giving a range of states (e.g. Mn shows +2 to +7).

Atomic and ionic radii trend

Radii fall across a series then level off due to increasing nuclear charge balanced by d-electron shielding; 4d and 5d elements have nearly equal size.

Coloured ions

Colour arises from d–d electronic transitions absorbing visible light; ions with d⁰ or d¹⁰ (Sc³⁺, Zn²⁺) are colourless as no such transition is possible.

Magnetic properties

Unpaired d-electrons make many ions paramagnetic; the spin-only moment μ = √[n(n+2)] BM estimates the number of unpaired electrons n.

Catalytic activity

Variable oxidation states and the ability to adsorb reactants on their surface let transition metals (Fe, Ni, V₂O₅, Pt) provide low-energy reaction paths.

Alloy and interstitial compounds

Similar radii let metals form alloys; small atoms (H, C, N) occupy lattice holes to give hard interstitial compounds like cementite and steel.

Complex formation

Small size, high charge and available d-orbitals let transition metal ions accept lone pairs from ligands and form coordination complexes.

Potassium dichromate

K₂Cr₂O₇, orange; a strong oxidiser in acid (Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O). Exists in equilibrium with yellow chromate at high pH.

Potassium permanganate

KMnO₄, purple; in acid MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (n=5), a very strong oxidising agent used in redox titrations.

Lanthanoid contraction

Steady decrease in size across the 4f lanthanoids from poor shielding by f-electrons; it makes Zr and Hf nearly identical in size and separates the elements' properties.

f-block elements

Lanthanoids (4f) show a common +3 state; actinoids (5f) show more variable states, are all radioactive, and have a similar actinoid contraction.

Enthalpy of atomisation

High for the middle of a series (many unpaired d-electrons give strong metallic bonding), explaining high melting points; low where the d-subshell is full or half-full.