The d- and f-Block Elements — Notes

The d-block occupies groups 3–12; its four series (3d, 4d, 5d, 6d) lie in periods 4–7. A transition element has an incompletely filled d subshell in its atom or in any of its common oxidation states; so Cu ($\ce{Cu^2+}$ is $3d^9$) is a transition element, while Zn, Cd and Hg ($d^{10}$ in atom and ion) are not.

General configuration $(n-1)d^{1\text{–}10}ns^{1\text{–}2}$. Because 3d and 4s energies are close and half-filled/filled d subshells are extra stable, Cr is $[\text{Ar}]3d^5 4s^1$ and Cu is $[\text{Ar}]3d^{10}4s^1$. On ionisation, 4s electrons are removed before 3d: $\ce{Fe^2+}$ $3d^6$, $\ce{Fe^3+}$ $3d^5$, $\ce{Mn^2+}$ $3d^5$, $\ce{Cu^2+}$ $3d^9$.

The f-block (lanthanoids and actinoids) is placed separately below the table; these are the inner transition elements.

• Physical: hard, dense, high melting points (except Zn, Cd, Hg); unpaired d electrons strengthen metallic bonding, so melting point and enthalpy of atomisation peak mid-series.
• Radii: fall at first, then nearly constant across a series (poor shielding by 3d balanced by d–d repulsion); 4d and 5d radii almost equal (lanthanoid contraction).
• Ionisation enthalpy: rises slowly and irregularly; high second IE for Cr and Cu (stable $d^5$, $d^{10}$ of $\ce{M+}$).
• Oxidation states: variable, usually differing by one; maximum +7 at Mn; high states in oxides/fluorides.
• E° values: $E^\circ(\ce{M^2+/M})$ negative except Cu (+0.34 V): copper does not displace hydrogen from dilute acids. $\ce{Mn^3+}$ ($E^\circ$ +1.57 V) and $\ce{Co^3+}$ (+1.97 V) are strong oxidants; $\ce{Cr^2+}$ ($E^\circ(\ce{Cr^3+/Cr^2+}) = -0.41$ V) is a reductant.
• Magnetism: paramagnetic if unpaired electrons; spin-only $\mu = \sqrt{n(n+2)}$ BM.
• Colour: d–d transitions absorb visible light; $d^0$ ($\ce{Sc^3+}$, $\ce{Ti^4+}$) and $d^{10}$ ($\ce{Zn^2+}$, $\ce{Cu+}$) ions are colourless.
• Catalysis: variable oxidation states and surface adsorption (Fe in the Haber process, $\ce{V2O5}$ in the contact process, Ni in hydrogenation).
• Interstitial compounds: small atoms (H, C, N) in lattice holes, e.g. TiC, $\ce{Mn4N}$, $\ce{Fe3H}$: hard, high melting, chemically inert, metallic conductivity.
• Alloys: similar atomic radii allow substitution (brass, bronze, stainless steel). Complexes: small, highly charged ions with vacant d orbitals.

K₂Cr₂O₇ preparation: $\ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2}$; $\ce{2Na2CrO4 + 2H+ -> Na2Cr2O7 + 2Na+ + H2O}$; $\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}$. Chromate (yellow, tetrahedral) and dichromate (orange, two tetrahedra sharing an O) interconvert with pH; Cr is +6 in both.

Oxidising action (acid): $\ce{Cr2O7^2- + 14H+ + 6e- -> 2Cr^3+ + 7H2O}$ ($E^\circ$ +1.33 V): oxidises $\ce{Fe^2+}$ to $\ce{Fe^3+}$, $\ce{I-}$ to $\ce{I2}$, $\ce{H2S}$ to S, $\ce{Sn^2+}$ to $\ce{Sn^4+}$; orange → green.

KMnO₄ preparation: $\ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O}$; manganate (green, $d^1$, paramagnetic) disproportionates in acid, $\ce{3MnO4^2- + 4H+ -> 2MnO4- + MnO2 + 2H2O}$, or is oxidised electrolytically. Permanganate is purple, tetrahedral, diamagnetic (colour from charge transfer).

Oxidising action: acid $\ce{MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}$ ($E^\circ$ +1.52 V) — oxidises $\ce{Fe^2+}$, oxalate, iodide; it is a self-indicator. Neutral/faintly alkaline $\ce{MnO4- + 2H2O + 3e- -> MnO2 + 4OH-}$, e.g. iodide → iodate. Do not acidify with HCl (chloride is oxidised).

Titration ratios: $\ce{Cr2O7^2-} : \ce{Fe^2+} = 1 : 6$; $\ce{MnO4-} : \ce{Fe^2+} = 1 : 5$; $\ce{MnO4-} : \ce{C2O4^2-} = 2 : 5$.

Lanthanoids (Ce–Lu) fill 4f: $[\text{Xe}]4f^{1\text{–}14}5d^{0\text{–}1}6s^2$. Common oxidation state +3; $\ce{Ce^4+}$ ($4f^0$) is an oxidant, $\ce{Eu^2+}$ ($4f^7$) and $\ce{Yb^2+}$ ($4f^{14}$) are reductants. Only promethium is radioactive.

Lanthanoid contraction: steady decrease in atomic and ionic radii from La to Lu, due to the imperfect shielding of one 4f electron by another. Consequences: 4d and 5d elements of a group have almost identical radii (Zr 160 pm, Hf 159 pm) and properties, so they are hard to separate; basicity of $\ce{Ln(OH)3}$ decreases from La to Lu; lanthanoids are hard to separate from each other.

Actinoids (Th–Lr) fill 5f. 5f, 6d and 7s energies are close and 5f orbitals extend further than 4f, so actinoids show more oxidation states (up to +7 for Np, Pu), a greater actinoid contraction, more complex formation, and all are radioactive.

• Iron and steels: construction; stainless steel (Fe with Cr and Ni) resists corrosion.
• Titanium: light, strong alloys for aircraft and medical implants.
• Catalysts: $\ce{V2O5}$ (contact process), Fe (Haber process), Ni (hydrogenation of oils), $\ce{TiCl4}$ with $\ce{Al(CH3)3}$ (Ziegler catalyst for polythene), $\ce{PdCl2}$ (Wacker process).
• Coinage and electrical metals: Cu, Ag, Au; AgBr in photography.
• Mischmetall: about 95% lanthanoid metals with about 5% iron (and traces of S, C, Ca, Al) — lighter flints, alloys.
• Lanthanoid oxides: phosphors in screens and lamps.
• Actinoids: uranium and plutonium as nuclear fuels.