The d- and f-Block Elements covers the position and electronic configuration of transition elements, periodic trends in size, density and melting point, variable oxidation states, magnetic moment, colour and catalytic behaviour, the important oxidisers potassium permanganate and potassium dichromate, and the f-block lanthanoids and actinoids with lanthanoid contraction. It is a high-yield NEET chapter where questions test electronic configurations, the spin-only magnetic moment, the reasons for colour, and lanthanoid contraction.
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Who Are the d-Block Elements?
Transition elements sit in the middle of the table (groups 3-12) where the inner (n-1)d orbitals fill up.
True transition element = partially filled d in element OR a common ion.
- 4 series: 3d (Sc→Zn), 4d, 5d, 6d (incomplete)
- Zn, Cd, Hg (d10s2) are NOT true transition metals
- ns electrons leave before (n-1)d → Fe2⁺ is 3d6
Cr and Cu Configurations
Half-filled and fully-filled d-subshells are extra stable, so Cr and Cu break the simple filling rule.
Not 3d44s2 / 3d94s2 — the lone 4s electron buys d5 / d10 stability.
- Cr → half-filled 3d5 is stable
- Cu → fully-filled 3d10 is stable
- Unpaired d-electrons drive colour, magnetism, oxidation states
Size, Density & Melting Point
Added electrons enter an inner d-shell, so across a series atomic radius stays almost constant.
Os and Ir are the densest metals; Mn, Tc dip in MP (stable d5).
- Radius: small drop, flat middle, slight rise at Cu, Zn
- Very high MP from strong ns + (n-1)d metallic bonding
- High atomisation enthalpy → good catalysts and alloys
Variable Oxidation States
ns and (n-1)d energies are very close, so a varying number of electrons can bond.
Highest state lives in the oxide/fluoride: Mn2O7, KMnO4, OsO4 (+8).
- +2 grows more stable across the series (Mn2⁺ = stable d5)
- O and F stabilise the highest states
- Sc shows only +3; Zn shows only +2
Magnetic Moment
Paramagnetism comes from unpaired d-electrons; count them and plug into the spin-only formula.
n = unpaired e⁻. n=0 → diamagnetic. Unit is Bohr Magneton (BM).
- More unpaired e⁻ → more paramagnetic
- Fe3⁺ (3d5, n=5) → μ = √35 ≈ 5.9 BM
- Ignores orbital contribution (spin-only approximation)
Colour, Catalysis & Complexes
d-d electronic transitions absorb visible light, so most transition-metal ions are coloured.
Sc3⁺, Ti4⁺ (d0) and Zn2⁺, Cu⁺ (d10) → no d-d jump → colourless.
- Catalysts (V2O5, Fe, Ni, Pt) via variable states / adsorption
- Form complexes and interstitial compounds (H, C, N in lattice)
- Similar atomic sizes → form alloys (e.g. steel)
Lanthanoids & Actinoids
Inner transition elements fill the deeper (n-2)f orbitals and sit below the main table.
Characteristic state +3; actinoids reach +6/+7 (close 5f, 6d, 7s).
- Lanthanoids: dominant +3 state
- Eu, Yb → +2 and Ce, Tb → +4 (half/full 4f link)
- Ce4⁺ is a strong oxidising agent
Lanthanoid Contraction
Across the lanthanoids, atomic and ionic size steadily shrinks because 4f electrons shield poorly.
Pairs Zr/Hf and Nb/Ta become almost identical and hard to separate.
- Rising effective nuclear charge pulls electrons inward
- 2nd and 3rd transition series end up similar in size
- Basicity differences of later lanthanoids aid separation
KMnO4 & K2Cr2O7
Two NEET-staple oxidisers — learn their colour, structure and acidic-medium half-reactions.
KMnO4: Mn(+7), purple, tetrahedral MnO4⁻. K2Cr2O7: Cr(+6), orange.
- Acidic medium: MnO4⁻ takes 5e⁻, Cr2O72⁻ takes 6e⁻
- Neutral/basic MnO4⁻ → 3e⁻ → MnO2 (brown)
- CrO42⁻ (yellow, basic) ⇌ Cr2O72⁻ (orange, acidic)
📝 Practice The d- and f-Block Elements — 10 NEET PYQs
Real previous-year questions · with answers & solutions
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Related Chapters in Class 12 Chemistry
- Coordination Compounds Class 12 Notes
- The p-Block Elements Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
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Frequently Asked Questions
A transition element is one that has a partially filled d-subshell either in its elementary (ground) state or in one of its common oxidation states. By this definition zinc, cadmium and mercury are not regarded as true transition elements because their d-subshell is completely filled (d10).
The spin-only magnetic moment equals the square root of n times (n plus 2) Bohr magnetons, where n is the number of unpaired electrons. More unpaired electrons means more paramagnetic, while an ion with no unpaired electrons is diamagnetic. For example, the Fe3+ ion with five unpaired electrons has a moment of about 5.9 Bohr magnetons.
Most transition-metal ions are coloured because of d-d transitions, where an electron jumps between d-orbitals split into different energy levels, absorbing part of visible light. Ions with empty d-orbitals (d0, like Sc3+) or completely filled d-orbitals (d10, like Zn2+) are colourless because no d-d transition is possible.
Lanthanoid contraction is the steady decrease in atomic and ionic size across the lanthanoid series, caused by the poor shielding of the nucleus by 4f electrons so that increasing nuclear charge pulls the electrons inward. A key consequence is that zirconium and hafnium have almost identical sizes and are very difficult to separate.
Manganese has five 3d and two 4s electrons available, and because the energies of the 3d and 4s electrons are very close, all of them can take part in bonding. This allows manganese to show oxidation states from +2 up to +7, the widest range in the 3d series.