The fascinating world of transition metals — from the colourful KMnO₄ to the magnetic iron core of the Earth. With interactive magnetic moment calculator, electronic configurations, oxidation states, and 20+ real-life examples.
Before diving in, here's the entire chapter on one page. How position relates to configuration, which explains properties, oxidation states, and colours.
d-block elements have partially filled (n−1)d orbitals. This single feature drives their characteristic properties: variable oxidation states, colour, magnetism, catalytic activity, complex formation, and alloy formation. Lanthanoids and actinoids (f-block) have similar but more complex behaviour.
d-block elements: The elements in which the last electron enters the (n−1)d orbital.
Groups: 3 to 12 (10 groups, 10 elements each → 3 series)
Periods: 4th (3d series), 5th (4d series), 6th (5d series), 7th (6d series, incomplete)
f-block: Placed separately at the bottom — lanthanoids (4f) and actinoids (5f).
Transition metals: Elements having partially filled d-orbitals in their ground state OR in their commonly occurring oxidation states.
Exceptions:
| Series | Period | Elements | Config | Notes |
|---|---|---|---|---|
| 3d | 4 | Sc to Zn (21-30) | 3d¹⁻¹⁰ 4s¹⁻² | First transition series |
| 4d | 5 | Y to Cd (39-48) | 4d¹⁻¹⁰ 5s¹⁻² | Second transition series |
| 5d | 6 | La to Hg (57-80) | 5d¹⁻¹⁰ 6s¹⁻² | Third transition series (with 4f filled in between) |
| 6d | 7 | Ac to Cn (89-112) | 6d¹⁻¹⁰ 7s¹⁻² | Fourth, incomplete |
d-block = 10 groups (3-12) × 3-4 periods. Transition metals = have partially filled d orbitals (in atom or common oxidation state). Zn, Cd, Hg are exceptions (d¹⁰ always). f-block (lanthanoids + actinoids) placed separately.
d-block: (n−1)d¹⁻¹⁰ ns¹⁻²
f-block: (n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns²
Key point: ns orbital fills BEFORE (n−1)d because ns has lower energy (n + l rule). But once d starts filling, s electrons can shift to d to attain stable configurations.
Cr (Z = 24): [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²)
Cu (Z = 29): [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²)
Why? Half-filled (d⁵) and completely filled (d¹⁰) configurations are more stable due to:
Other anomalies: Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au — all have anomalous configs.
| Element | Expected | Actual | Reason |
|---|---|---|---|
| Cr (24) | 3d⁴ 4s² | 3d⁵ 4s¹ | Half-filled stability |
| Cu (29) | 3d⁹ 4s² | 3d¹⁰ 4s¹ | Fully-filled stability |
| Nb (41) | 4d³ 5s² | 4d⁴ 5s¹ | Half-filled |
| Mo (42) | 4d⁴ 5s² | 4d⁵ 5s¹ | Half-filled |
| Ru (44) | 4d⁶ 5s² | 4d⁷ 5s¹ | Increased stability |
| Pd (46) | 4d⁸ 5s² | 4d¹⁰ 5s⁰ | Fully-filled |
| Ag (47) | 4d⁹ 5s² | 4d¹⁰ 5s¹ | Fully-filled |
Choose a 3d element — see its electronic configuration, oxidation states, and predicted magnetic moment.
General config: (n−1)d¹⁻¹⁰ ns¹⁻². Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹) are anomalous due to exchange energy and symmetry of half/fully-filled configurations. Other anomalies: Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au.
1. Metallic character: All are metals. High tensile strength, malleability, ductility. Due to strong metallic bonding (unpaired d electrons participate).
2. High melting and boiling points: Due to strong metallic bonds (interatomic bonding from unpaired d electrons). Max around middle (Cr, Mo, W — very high MP).
3. Enthalpy of atomization: High, peaks around middle of series (max unpaired electrons → strongest bonds).
4. Atomic/ionic radii: Decrease across series (increased nuclear charge, d electrons shield poorly). Then increase slightly at end.
5. Density: High — atoms packed closely.
6. Ionization enthalpy: Increases across series (but not smoothly — anomalies due to electron exchange).
7. Electronegativity: Increases across series.
8. Oxidation states: Variable (see next section).
9. Colour: Ions usually coloured (d-d transitions).
10. Magnetic properties: Usually paramagnetic (unpaired electrons).
11. Catalytic properties: Many are catalysts (variable oxidation states + surface adsorption).
12. Complex formation: Small, highly charged ions with vacant d orbitals → good Lewis acids → form coordination complexes.
13. Interstitial compounds: Small atoms (H, C, N) fit in interstitial gaps → hard, high MP, retain metallic properties.
14. Alloy formation: Similar atomic sizes → form substitutional alloys (brass, bronze, steel).
Transition metals: hard, dense, high MP, metallic. All properties stem from partially filled d orbitals: variable oxidation states, colour, magnetism, catalytic activity, complex formation, interstitial compounds, alloy formation.
Because (n−1)d and ns orbitals have similar energies, transition metals can lose electrons from both orbitals, giving multiple oxidation states.
Common pattern: +2 is common (loss of 2 ns electrons). Higher oxidation states require loss of some d electrons too.
Maximum oxidation state: Increases up to Mn (+7), then decreases (d electrons paired)
| Element | Common Oxidation States | Most Stable |
|---|---|---|
| Sc | +3 | +3 |
| Ti | +2, +3, +4 | +4 |
| V | +2, +3, +4, +5 | +5 |
| Cr | +2, +3, +6 | +3 (and +6 in compounds) |
| Mn | +2, +3, +4, +6, +7 | +2 (and +7 in KMnO₄) |
| Fe | +2, +3 | +2, +3 |
| Co | +2, +3 | +2 (and +3 in complexes) |
| Ni | +2 | +2 |
| Cu | +1, +2 | +2 |
| Zn | +2 | +2 |
Mn has configuration 3d⁵ 4s². It can lose all 7 electrons (2 from 4s, 5 from 3d) to give Mn⁷⁺ (d⁰). But beyond Mn, protons increase → electrons held more tightly → harder to reach higher oxidation states. That's why Fe max is +6 (rare), Co +5 (rare), Ni +4 (in complexes).
Variable oxidation states due to similar energies of (n−1)d and ns. Max at Mn (+7). Common: Fe(+2,+3), Cu(+1,+2), Cr(+3,+6). Higher states are strong oxidizers. Lower states are often reducing agents.
Cause: d-d transitions. Electrons in partially filled d orbitals absorb visible light, excite to higher d levels. The complementary colour is observed.
Requirements:
d⁰ or d¹⁰ ions are colourless — no d-d transitions possible.
| Ion | Config | Colour | Example |
|---|---|---|---|
| Sc³⁺ | d⁰ | Colourless | ScCl₃ |
| Ti⁴⁺ | d⁰ | Colourless | TiO₂ |
| V⁵⁺ | d⁰ | Colourless | V₂O₅ (orange due to charge transfer) |
| Cr³⁺ | d³ | Green | CrCl₃ |
| Cr⁶⁺ | d⁰ | Orange (charge transfer) | K₂Cr₂O₇ |
| Mn²⁺ | d⁵ | Pale pink | MnSO₄ |
| Mn⁷⁺ | d⁰ | Purple (charge transfer) | KMnO₄ |
| Fe²⁺ | d⁶ | Pale green | FeSO₄ |
| Fe³⁺ | d⁵ | Yellow-brown | FeCl₃ |
| Co²⁺ | d⁷ | Pink | CoCl₂·6H₂O |
| Ni²⁺ | d⁸ | Green | NiSO₄ |
| Cu²⁺ | d⁹ | Blue | CuSO₄·5H₂O |
| Zn²⁺ | d¹⁰ | Colourless | ZnSO₄ |
Paramagnetism: Due to unpaired electrons. Attracted by magnetic field. Most transition metal ions are paramagnetic.
Diamagnetism: All paired electrons. Weakly repelled by magnetic field. d⁰ and d¹⁰ ions are diamagnetic.
Ferromagnetism: Strong permanent magnetism (Fe, Co, Ni). Due to aligned spins in domains.
Antiferromagnetism: Adjacent spins antiparallel, net zero (MnO).
Spin-only magnetic moment formula:
μ = √n(n+2) BM
Where n = number of unpaired electrons, BM = Bohr Magneton.
| n (unpaired e⁻) | μ (BM) | Example |
|---|---|---|
| 0 | 0 | Zn²⁺ (d¹⁰), Sc³⁺ (d⁰) |
| 1 | 1.73 | Ti³⁺ (d¹), Cu²⁺ (d⁹) |
| 2 | 2.83 | V³⁺ (d²), Ni²⁺ (d⁸) |
| 3 | 3.87 | Cr³⁺ (d³), Co²⁺ (d⁷) |
| 4 | 4.90 | Mn³⁺ (d⁴), Fe²⁺ (d⁶) |
| 5 | 5.92 | Fe³⁺ (d⁵), Mn²⁺ (d⁵) |
Enter number of unpaired electrons (or choose ion) — calculate magnetic moment.
Colour = d-d transitions (or charge transfer). d⁰ and d¹⁰ ions are colourless. Magnetic moment μ = √n(n+2) BM. Max moment with 5 unpaired electrons (5.92 BM for Mn²⁺, Fe³⁺).
Transition metal ions form coordination complexes with ligands (H₂O, NH₃, Cl⁻, CN⁻, etc.) because:
Examples: [Cu(NH₃)₄]²⁺ (deep blue), [Fe(CN)₆]³⁻ (red), [Ni(CO)₄] (colourless), [Co(NH₃)₆]³⁺ (yellow-orange).
Transition metals are excellent catalysts because:
Examples:
Small atoms (H, C, N, B) occupy interstitial spaces in transition metal lattices.
Properties: Hard, high MP, retain metallic conductivity, chemically inert
Examples: TiC, Fe₃H, Mn₄N, WC (tungsten carbide), steel (Fe + C)
Applications: Steel (C in Fe), tungsten carbide (cutting tools), TiN (hard coatings).
Transition metals have similar atomic sizes → form substitutional alloys.
Examples:
Transition metals form complexes (vacant d, high charge), act as catalysts (variable oxidation states, surface adsorption), form interstitial compounds (hard, high MP), and form alloys (similar atomic sizes). All properties follow from partially filled d orbitals.
Preparation: From MnO₂ (pyrolusite)
Structure: Purple crystals, Mn in +7 state (d⁰), tetrahedral MnO₄⁻ ion
Colour: Purple — due to charge transfer (O²⁻ → Mn⁷⁺), NOT d-d (no d electrons)
Properties: Strong oxidizing agent in acidic, alkaline, and neutral media:
Preparation: From chromite ore (FeCr₂O₄)
Structure: Orange crystals, Cr in +6 state (d⁰), dichromate Cr₂O₇²⁻ ion
Colour: Orange — charge transfer (O²⁻ → Cr⁶⁺), NOT d-d
Properties: Strong oxidizing agent in acidic medium:
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V) — green Cr³⁺
| Property | KMnO₄ | K₂Cr₂O₇ |
|---|---|---|
| Colour | Purple | Orange |
| Metal | Mn (+7, d⁰) | Cr (+6, d⁰) |
| Media | Acidic, neutral, alkaline | Mainly acidic |
| Acidic product | Mn²⁺ (pale pink) | Cr³⁺ (green) |
| E° (acidic) | +1.51 V (stronger) | +1.33 V |
| Titration use | Self-indicator | External indicator (diphenylamine) |
KMnO₄: purple, Mn(+7, d⁰), strongest oxidizer in acid (+1.51 V). K₂Cr₂O₇: orange, Cr(+6, d⁰), strong oxidizer (+1.33 V). Both work in multiple media with different products. Colour due to charge transfer, not d-d.
Elements: La (57) to Lu (71) — 14 elements after lanthanum
Electronic configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
Common: Ce (4f¹5d⁰6s² → 4f¹) — actually Ce is [Xe]4f¹5d¹6s² to help fill f-block
Common oxidation state: +3 (Ln³⁺)
Definition: Steady decrease in atomic and ionic radii from La to Lu due to poor shielding of 4f electrons.
Cause: As we move across, 4f electrons increase. But 4f orbitals are diffuse and shield the nucleus poorly. Effective nuclear charge increases → electrons pulled closer → smaller radii.
Consequences:
Lanthanoids: 14 elements (La to Lu, though officially Lu follows). Common +3 state. Lanthanoid contraction due to poor 4f shielding. +2 and +4 states exist for stable f⁰, f⁷, f¹⁴. Applications: Nd magnets, Eu phosphors, Ce polishing, La lenses.
Elements: Ac (89) to Lr (103) — 14 elements after actinium
Electronic configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
Common oxidation states: +3 (most common), +4, +5, +6, +7 (more varied than lanthanoids)
| Property | Lanthanoids (4f) | Actinoids (5f) |
|---|---|---|
| Filling orbital | 4f | 5f |
| Common ox. state | +3 | +3 (but more varied) |
| Other ox. states | +2, +4 (rare) | +2 to +7 (common) |
| Shielding | Poor (4f) | Even poorer (5f more diffuse) |
| Contraction | Strong (lanthanoid contraction) | Stronger (actinoid contraction) |
| Radioactivity | Non-radioactive (except Pm) | All radioactive |
| Colour | Many coloured | Many coloured |
| Complex formation | Limited | More (higher charge, smaller size) |
| Basic character | Less basic | More basic |
5f orbitals are more diffuse and at higher energy than 4f. Also, 5f electrons shield poorly → high effective nuclear charge → but relatively more accessible energy levels. Result: 5f electrons are more easily lost AND 6d electrons also participate.
Early actinoids (Th, Pa, U): Can reach high oxidation states like +4, +5, +6 (e.g., UO₂²⁺ — uranyl).
Later actinoids (Am, Cm, etc.): +3 more common (like lanthanoids).
Actinoids: 14 elements (Ac to Lr), all radioactive. 5f filling. More varied oxidation states (+3 to +7) due to poorly shielded 5f. Higher complex formation tendency. Applications: nuclear fuel (U, Pu), smoke detectors (Am), RTGs (Pu-238).
"Sc Ti V Cr Mn Fe Co Ni Cu Zn" — say as "Scatty vikram man fay co ni kuzn"
Sc(21) Ti(22) V(23) Cr(24) Mn(25) Fe(26) Co(27) Ni(28) Cu(29) Zn(30)
"Zinc, Cadmium, Mercury — FULL d, not transition"
They have d¹⁰ configuration in atom and +2 state → no partially filled d → NOT transition metals.
"Cr and Cu are Special — half-filled and fully-filled"
Cr = 3d⁵ 4s¹ (half-filled d), Cu = 3d¹⁰ 4s¹ (fully-filled d)
Both prefer extra stable d⁵/d¹⁰ over 4s².
"√n(n+2)" — "root n times n-plus-two"
n = unpaired electrons. Max 5 unpaired (d⁵) → μ = 5.92 BM
"d⁰ and d¹⁰ → colourless"
Transition metal ions with 0 or 10 d-electrons are colourless (Sc³⁺, Zn²⁺, Cu⁺).
d-d transitions need partially filled d orbitals
"Permanganate PURPLE, Dichromate ORANGE"
KMnO₄ — Mn(+7) — purple. K₂Cr₂O₇ — Cr(+6) — orange.
Both d⁰, colour due to charge transfer (not d-d).
"4f shields poorly — contraction results"
Lanthanoids get smaller from La to Lu because 4f electrons are diffuse and don't shield well.
"+4 for Ce (f⁰), +2 for Eu (f⁷), +2 for Yb (f¹⁴)"
Stable when reaches empty (f⁰), half-filled (f⁷), or fully filled (f¹⁴).
Use "Sc Ti V Cr Mn Fe Co Ni Cu Zn" for 3d series. "Cr and Cu are special" for anomalies. "√n(n+2)" for magnetic moment. "Permanganate PURPLE, Dichromate ORANGE" for colours.
Q1. Why is Zn not considered a transition metal?
Transition metal = element with partially filled d orbitals in atom or in common oxidation state.
Zn (Z=30): [Ar] 3d¹⁰ 4s². Zn²⁺: [Ar] 3d¹⁰.
Both have fully-filled d¹⁰ → no unpaired d electrons → NOT a transition metal.
Q2. Cr has configuration 3d⁵ 4s¹ instead of expected 3d⁴ 4s². Explain.
According to filling rules: [Ar] 3d⁴ 4s²
Actual: [Ar] 3d⁵ 4s¹
d⁵ (half-filled) has 5 parallel electrons. More exchanges = more stability. Exchange energy ∝ n(n-1)/2, where n = parallel electrons.
Half-filled (d⁵) and fully-filled (d¹⁰) configurations are symmetrically distributed → lower energy → more stable.
4s and 3d are close in energy, so electron can shift easily. The stability gain from d⁵ outweighs the small energy cost.
Q3. Calculate the magnetic moment of Fe³⁺ (d⁵).
Fe (Z=26): [Ar] 3d⁶ 4s². Fe³⁺: [Ar] 3d⁵.
d⁵ has 5 unpaired electrons (Hund's rule — all parallel).
μ = √n(n+2) BM
μ = √(5 × 7) = √35 = 5.92 BM
Q4. Why are Sc³⁺ and Zn²⁺ colourless while Cr³⁺ is green and Cu²⁺ is blue?
d-d transitions require partially filled d orbitals. Electrons absorb visible light and excite to higher d levels, giving complementary colour.
Sc³⁺ = d⁰ (no d electrons). Zn²⁺ = d¹⁰ (all paired). No d-d transitions possible → colourless.
Cr³⁺ = d³ (3 unpaired). Cu²⁺ = d⁹ (1 unpaired). Both have partially filled d → d-d transitions → coloured (green and blue).
Q5. Write the preparation of KMnO₄ from pyrolusite. Also, give its reactions in acidic, neutral, and alkaline media.
MnO₂ + 2KOH + ½O₂ → K₂MnO₄ + H₂O
(Pyrolusite + KOH + air → potassium manganate, green)
3K₂MnO₄ + 2CO₂ → 2KMnO₄ + MnO₂ + 2K₂CO₃
(or with Cl₂, H₂O — green manganate → purple permanganate)
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.51 V)
Purple → pale pink Mn²⁺
MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (E° = +1.23 V)
Purple → brown MnO₂
MnO₄⁻ + e⁻ → MnO₄²⁻ (E° = +0.56 V)
Purple → green MnO₄²⁻
Q6. Compare lanthanoids and actinoids with respect to electronic configuration, oxidation states, chemical reactivity, and radioactivity.
Lanthanoids: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
Actinoids: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
Lanthanoids: mainly +3 (rare +2, +4)
Actinoids: +3 to +7 (more varied)
Lanthanoids: quite reactive, silvery-white, tarnish rapidly
Actinoids: very reactive, especially early ones (more electropositive)
Both show contraction — actinoid contraction is greater due to poorer 5f shielding
Lanthanoids: non-radioactive (except Pm)
Actinoids: ALL radioactive — some very long-lived (U, Th), others short (many artificial)
Q7. Explain the following: (a) Transition metals form interstitial compounds, (b) KMnO₄ is coloured, (c) E°(Mn³⁺/Mn²⁺) is more positive than E°(Cr³⁺/Cr²⁺).
Small atoms (H, C, N, B) fit in the interstitial gaps of the metal lattice. This results in compounds that are:
Examples: steel (Fe+C), tungsten carbide (WC)
KMnO₄ contains Mn(+7) — d⁰ configuration. So no d-d transitions. Yet it's purple. The colour is due to ligand-to-metal charge transfer (LMCT): electrons from O²⁻ (ligand) absorb visible light and transfer to empty d orbitals of Mn⁷⁺. Since d orbitals are empty, charge transfer happens easily. Result: intense purple colour.
Mn³⁺/Mn²⁺: E° = +1.51 V
Cr³⁺/Cr²⁺: E° = −0.41 V
Mn³⁺ (d⁴) is unstable — it wants to become Mn²⁺ (d⁵, half-filled stable). So it easily accepts electron → strong oxidizing agent.
Cr³⁺ (d³) is very stable (half-filled t₂g in octahedral field) → doesn't want to gain electron → poor oxidizing agent.
Selected exercise questions from NCERT Class 12 Chemistry Chapter 4. Click each to reveal the solution.
(a) Cr³⁺: Cr = [Ar] 3d⁵ 4s¹. Cr³⁺ = [Ar] 3d³ (d³)
(b) Cu²⁺: Cu = [Ar] 3d¹⁰ 4s¹. Cu²⁺ = [Ar] 3d⁹ (d⁹)
(c) Fe³⁺: Fe = [Ar] 3d⁶ 4s². Fe³⁺ = [Ar] 3d⁵ (d⁵)
(d) Co²⁺: Co = [Ar] 3d⁷ 4s². Co²⁺ = [Ar] 3d⁷ (d⁷)
(e) Mn²⁺: Mn = [Ar] 3d⁵ 4s². Mn²⁺ = [Ar] 3d⁵ (d⁵)
Cr²⁺ (d⁴): Reducing agent. Cr²⁺ loses electron to become Cr³⁺ (d³). d³ is very stable (half-filled t₂g in octahedral field). So Cr²⁺ easily gets oxidized.
Mn³⁺ (d⁴): Oxidizing agent. Mn³⁺ gains electron to become Mn²⁺ (d⁵). d⁵ is very stable (half-filled). So Mn³⁺ easily gets reduced.
The difference is the final state: Cr³⁺ (d³) vs Mn²⁺ (d⁵). Both are stable, but the path to stability involves different electron movements.
Disproportionation: A reaction in which the same substance is simultaneously oxidized and reduced (from an intermediate oxidation state to two different states — one higher, one lower).
Example: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
Mn in +6 state (manganate) disproportionates to Mn(+7) in permanganate and Mn(+4) in MnO₂.
Another example: 2Cu⁺ → Cu²⁺ + Cu (Cu in +1 → +2 and 0)
Answer: This is not always true. For many metals, the highest oxidation state is observed in 4d or 5d series.
Examples:
Os (5d) = +8 (OsO₄) — highest known
Ru (4d) = +8 (RuO₄)
Mn (3d) = +7 (KMnO₄) — highest in 3d series
W (5d) = +6 (WO₃)
However, the maximum state = number of valence electrons (2 for 4s + 5 for 3d = 7 for Mn).
For 4d series: Y has 5 valence (5s² 4d¹), max +5. But heavier elements have more accessible d electrons.
Answer: In the first half of the 3d series (Sc to Mn), the number of d electrons increases from 1 to 5. The +2 state involves loss of 2 s electrons. As d count increases:
Sc²⁺ (d¹) — unstable, prefers +3 (d⁰)
Ti²⁺ (d²) — somewhat stable
V²⁺ (d³) — more stable (half-filled t₂g)
Cr²⁺ (d⁴) — stable
Mn²⁺ (d⁵) — most stable (half-filled d⁵)
Reason: exchange energy increases with number of parallel electrons. Half-filled configuration is most stable. So +2 state becomes more stable up to Mn²⁺ (d⁵).
Inner transition elements: f-block elements (lanthanoids and actinoids). Called "inner" because the f orbitals being filled are deeply buried inside the atom (below the outermost shell).
Lanthanoids: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s² (elements La to Lu, Z = 57-71)
Actinoids: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s² (elements Ac to Lr, Z = 89-103)
Note: The 4f/5f orbitals are filled progressively after the outermost s orbitals are filled. Some anomalies exist (Ce, Gd, Lu) due to stability of half/fully filled configurations.
Lanthanoid contraction: The steady decrease in atomic and ionic radii from La (57) to Lu (71) across the lanthanoid series.
Cause: As we move across the series, protons increase but the newly added 4f electrons shield the nucleus poorly (4f orbitals are diffuse). So effective nuclear charge increases → electrons pulled closer → radii decrease.
Consequences:
1. Zr (4d) and Hf (5d) have almost identical radii (due to lanthanoid contraction before Hf) — occur together in nature
2. Difficult to separate lanthanoids from each other — very similar properties
3. Basic character of Ln(OH)₃ decreases from La to Lu
(a) Electronic configuration: Lanthanoids: [Xe]4f¹⁻¹⁴5d⁰⁻¹6s². Actinoids: [Rn]5f¹⁻¹⁴6d⁰⁻¹7s².
(b) Oxidation state: Lanthanoids: +3 dominant (rare +2, +4). Actinoids: +3 to +7 — more varied.
(c) Atomic and ionic sizes: Both show contraction. Actinoid contraction is greater than lanthanoid contraction (poorer 5f shielding).
(d) Chemical reactivity: Both highly reactive. Actinoids more reactive than lanthanoids (larger atoms, more electropositive). Both react with water, acids, non-metals.
First series (3d): Smallest sizes, highest ionization enthalpy, most paramagnetic. Show widest range of oxidation states. Form stable complexes. Cr, Mn show maximum states (+6, +7).
Second (4d) and third (5d) series: Larger sizes, lower ionization enthalpy. Higher tendency to form high oxidation states (Ru +8, Os +8, W +6). More ability to form complexes (especially 5d). Formation of metal-metal bonds (Mo, W). Greater tendency to form polymeric halides. Colour and magnetism similar but more diffuse d orbitals give more complex spectra.
Similarities: Same general properties — metallic, high MP, variable oxidation states, colour, magnetism, catalysis, complex formation.
Interstitial compounds: Compounds formed when small atoms (H, C, N, B) occupy the interstitial spaces in the crystal lattice of a transition metal.
Why transition metals? Transition metal atoms are large enough that their crystal lattice has empty spaces (interstices) big enough to accommodate small non-metal atoms. Also, their metallic bonding is flexible enough that the small atoms don't disrupt the lattice completely.
Properties: Hard, high melting point, retain electrical conductivity, chemically inert.
Examples: Steel (Fe+C), tungsten carbide (WC), titanium nitride (TiN), manganese nitride (Mn₄N), vanadium hydride (VH₀.₅₆).
Uses: Steel for construction, carbides for cutting tools, TiN for hard coatings on drills.
| Cr³⁺ colour | Green | d³ configuration |
| Mn²⁺ colour | Pale pink | d⁵ configuration |
| Fe²⁺ colour | Pale green | d⁶ configuration |
| Fe³⁺ colour | Yellow-brown | d⁵ configuration |
| Co²⁺ colour | Pink | d⁷ configuration |
| Ni²⁺ colour | Green | d⁸ configuration |
| Cu²⁺ colour | Blue | d⁹ configuration |
| Zn²⁺ colour | Colourless | d¹⁰ configuration |
| KMnO₄ colour | Purple | Mn(+7), d⁰, charge transfer |
| K₂Cr₂O₇ colour | Orange | Cr(+6), d⁰, charge transfer |
| CuSO₄·5H₂O | Blue | Cu²⁺ (d⁹) aqua complex |
| Fe³⁺ max μ | 5.92 BM | 5 unpaired electrons |
| n | Number of unpaired electrons |
| μ | Magnetic moment (BM) |
| BM | Bohr Magneton |
| Ln | Lanthanoid (general) |
| An | Actinoid (general) |
| d-block | Groups 3-12 |
| f-block | Lanthanoids + Actinoids |
| LMCT | Ligand-to-Metal Charge Transfer |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key formulas & facts at one place — perfect for quick revision before exams.