1. Position, occurrence and electronic configuration
The d-block lies in groups 3–12, periods 4–7: 3d (Sc–Zn), 4d (Y–Cd), 5d (La, Hf–Hg), 6d (Ac, Rf–Cn). A transition element has an incompletely filled d subshell in its atom or in a common ion; Zn, Cd and Hg (d¹⁰ throughout) are not typical transition elements. Most 3d metals occur as oxide, sulfide or carbonate ores (haematite Fe₂O₃, chromite FeCr₂O₄, pyrolusite MnO₂, ilmenite FeTiO₃, copper pyrites CuFeS₂, zinc blende ZnS).
General configuration ; Cr 3d⁵4s¹, Cu 3d¹⁰4s¹ (extra stability of d⁵ and d¹⁰). Heavier series show more irregularities: Pd 4d¹⁰5s⁰, W 5d⁴6s², Pt 5d⁹6s¹. Ions lose ns electrons first (Fe²⁺ 3d⁶, Fe³⁺ 3d⁵).
Key terms
- Transition element
An element whose atom or one of whose common ions has an incompletely filled d subshell.
- d-block
The block of elements in which the last electron enters an (n−1)d subshell.
2. Metallic character, radii and ionisation enthalpy
All are metals; most are hard, dense and high-melting because unpaired d electrons add to metallic bonding; melting points and enthalpies of atomisation peak mid-series; Zn, Cd and Hg are soft and low-melting.
Radii: fall at the start of a series, then stay nearly constant (extra nuclear charge balanced by shielding and d–d repulsion). M²⁺ radii decrease gradually. 4d > 3d, but 4d ≈ 5d (Zr 160, Hf 159 pm) — the lanthanoid contraction.
Ionisation enthalpy: rises slowly and irregularly. High where d⁵ or d¹⁰ is broken: ΔiH₂ of Cr and Cu, ΔiH₃ of Mn and Zn. ΔiH₁ of Zn is high (3d¹⁰4s²).
- Metallic radius, 3d series
- ΔiH₁, 3d series
Key terms
- Lanthanoid contraction
The steady decrease in size across the lanthanoids, which makes 5d elements almost the same size as the 4d elements above them.
- Enthalpy of atomisation
The enthalpy needed to convert one mole of a metal into gaseous atoms; a measure of metallic bonding.
3. Oxidation states and variable valency
(n−1)d and ns energies are close, so several oxidation states occur, often differing by one. Maximum rises to +7 at Mn (sum of 3d and 4s electrons) and then falls as d electrons pair. Highest states appear in oxides, fluorides and oxoanions (Mn₂O₇, VF₅, CrO₄²⁻); zero state in carbonyls (Ni(CO)₄). Lower oxides are basic (MnO), higher ones acidic (Mn₂O₇).
Stability: d⁰, d⁵ and d¹⁰ favoured — Sc³⁺ (d⁰) only; Fe³⁺ (d⁵) more stable than Fe²⁺; Mn²⁺ (d⁵) more stable than Mn³⁺, so Mn³⁺ is oxidising; Cr²⁺ is reducing (to d³ Cr³⁺). In water Cu²⁺ is more stable than Cu⁺ because of the large hydration enthalpy of Cu²⁺.
| Ion | Configuration | Behaviour |
|---|---|---|
| Sc³⁺ | d⁰ | Only common state |
| Fe²⁺ | d⁶ | Oxidised in air to d⁵ Fe³⁺ |
| Mn³⁺ | d⁴ | Oxidising, goes to d⁵ Mn²⁺ |
| Cr²⁺ | d⁴ | Reducing, goes to d³ Cr³⁺ |
Key terms
- Variable valency
The ability of an element to show more than one oxidation state in its compounds.
4. Colour and magnetic properties
Colour: ligands split the d orbitals; a d electron absorbs visible light and is promoted (d–d transition); the ion shows the complementary colour. d⁰ (Sc³⁺, Ti⁴⁺) and d¹⁰ (Zn²⁺, Cu⁺) ions are colourless. Anhydrous CuSO₄ is white; the hydrate is blue.
Magnetism: unpaired electrons → paramagnetic; all paired → diamagnetic; Fe, Co, Ni are ferromagnetic. Spin-only moment BM. Fill five d orbitals singly before pairing.
| n | μ / BM |
|---|---|
| 1 | 1.73 |
| 2 | 2.83 |
| 3 | 3.87 |
| 4 | 4.90 |
| 5 | 5.92 |
Formulas
- Spin-only magnetic moment
Key terms
- d–d transition
Promotion of a d electron from a lower to a higher set of split d orbitals by absorption of visible light.
- Paramagnetism
Attraction into a magnetic field, caused by unpaired electrons.
- Diamagnetism
Weak repulsion by a magnetic field, shown when all electrons are paired.
5. Catalysis, complexes, interstitial compounds and alloys
Catalysts because of (i) variable oxidation states (unstable intermediates, e.g. Fe³⁺/Fe²⁺ in the iodide–peroxodisulfate reaction) and (ii) surfaces that adsorb reactants. Examples: Fe (Haber), V₂O₅ (contact), Ni (hydrogenation of oils), Pt/Rh (Ostwald).
Complexes: small, highly charged ions with vacant d orbitals of suitable energy accept lone pairs: [Cu(NH₃)₄]²⁺, [Fe(CN)₆]⁴⁻.
Interstitial compounds: H, B, C or N in lattice holes (TiC, Mn₄N, Fe₃H); hard, high-melting, metallic conductors, inert, often non-stoichiometric.
Alloys: similar atomic radii allow substitution: stainless steel (Fe, Cr, Ni), brass (Cu, Zn), bronze (Cu, Sn), nichrome (Ni, Cr).
| Property | Cause |
|---|---|
| Catalysis | Variable states; adsorption |
| Complex formation | Small, charged ions; vacant d orbitals |
| Interstitial compounds | Holes in lattice |
| Alloys | Similar radii |
Key terms
- Interstitial compound
A compound formed when small atoms occupy holes in a metal lattice.
- Alloy
A homogeneous mixture of a metal with other metals or non-metals.
Exam tips
Common mistakes
- Removing 3d electrons before 4s when writing configurations of ions.
- Writing Cr as 3d⁴4s² or Cu as 3d⁹4s².
- Forgetting the square root in the spin-only formula, or pairing electrons before all five d orbitals are singly filled.
- Calling Zn²⁺ or Sc³⁺ coloured.
How marks are usually awarded
- Reason questions (2 marks): the configuration involved and the stability argument (d⁰, d⁵ or d¹⁰).
- Magnetic moment (2–3 marks): configuration, number of unpaired electrons, substitution in √(n(n+2)).
- Catalysis (2 marks): both reasons with an example.